diff --git a/src/components/labs/__tests__/lab-unit-input.browser.test.tsx b/src/components/labs/__tests__/lab-unit-input.browser.test.tsx
new file mode 100644
index 000000000..d11dff4cf
--- /dev/null
+++ b/src/components/labs/__tests__/lab-unit-input.browser.test.tsx
@@ -0,0 +1,56 @@
+import { useState } from "react";
+import { describe, expect, it } from "vitest";
+import { userEvent } from "vitest/browser";
+
+import { renderWithApp } from "../../../../tests/browser/render";
+import { LabUnitInput } from "../lab-unit-input";
+
+/**
+ * The unit field as a form holds it, typed into key by key. A unit that
+ * starts with a 1 (10^9/L, 1/min) passes through "1" on the way, and that
+ * keystroke must not turn into the "no unit" choice.
+ */
+function Controlled({ initial = "" }: { initial?: string }) {
+  const [unit, setUnit] = useState(initial);
+  return (
+    <>
+      <label htmlFor="u">Unit</label>
+      <LabUnitInput id="u" value={unit} onChange={setUnit} />
+      <output data-testid="unit">{unit}</output>
+    </>
+  );
+}
+
+describe("<LabUnitInput> typed into", () => {
+  it("keeps a unit that starts with a 1", async () => {
+    const { screen } = await renderWithApp(<Controlled />);
+    await userEvent.type(screen.getByLabelText("Unit"), "10^9/L");
+    await expect
+      .element(screen.getByTestId("unit"))
+      .toHaveTextContent("10^9/L");
+    await expect
+      .element(screen.getByRole("checkbox", { name: /no unit/i }))
+      .not.toBeChecked();
+  });
+
+  it("shows a stored unity as the ticked choice and gives the field back on unticking", async () => {
+    const { screen } = await renderWithApp(<Controlled initial="1" />);
+    const box = screen.getByRole("checkbox", { name: /no unit/i });
+    await expect.element(box).toBeChecked();
+    await expect.element(screen.getByLabelText("Unit")).toBeDisabled();
+    await box.click();
+    await expect.element(screen.getByLabelText("Unit")).toBeEnabled();
+    await userEvent.type(screen.getByLabelText("Unit"), "mmol/L");
+    await expect
+      .element(screen.getByTestId("unit"))
+      .toHaveTextContent("mmol/L");
+  });
+
+  it("sets the unity when the box is ticked", async () => {
+    const { screen } = await renderWithApp(<Controlled initial="mg/dL" />);
+    await screen.getByRole("checkbox", { name: /no unit/i }).click();
+    await expect
+      .poll(() => screen.getByTestId("unit").element().textContent)
+      .toBe("1");
+  });
+});
diff --git a/src/components/labs/lab-unit-input.tsx b/src/components/labs/lab-unit-input.tsx
index cdbc5d1df..7545f791e 100644
--- a/src/components/labs/lab-unit-input.tsx
+++ b/src/components/labs/lab-unit-input.tsx
@@ -1,6 +1,6 @@
 "use client";
 
-import { useId, useRef } from "react";
+import { useId, useRef, useState } from "react";
 
 import { Checkbox } from "@/components/ui/checkbox";
 import { Input } from "@/components/ui/input";
@@ -48,8 +48,9 @@ interface LabUnitInputProps {
  * The unit of a lab value, with an explicit "no unit" choice for a ratio or an
  * index. Ticking it sets the unit to the UCUM unity `"1"`, which every write
  * path accepts as a real unit; leaving the field empty is still refused, so a
- * unit an extraction dropped is never taken for a value that has none. Typing
- * `1` by hand is the same choice and ticks the box.
+ * unit an extraction dropped is never taken for a value that has none. Only
+ * the box makes the choice: a unit typed key by key passes through "1" on its
+ * way to `10^9/L`, and that keystroke stays text.
  */
 export function LabUnitInput({
   id,
@@ -64,7 +65,9 @@ export function LabUnitInput({
   const { t } = useTranslations();
   const noneId = useId();
   const hintId = `${noneId}-hint`;
-  const none = allowNoUnit && isDimensionlessUnit(value);
+  // Set when a keystroke, not the box, left the field at "1".
+  const [typedOne, setTypedOne] = useState(false);
+  const none = allowNoUnit && isDimensionlessUnit(value) && !typedOne;
   // What was typed before the box was ticked, so unticking gives it back.
   const typed = useRef(none ? "" : value);
 
@@ -74,6 +77,7 @@ export function LabUnitInput({
       value={none ? "" : value}
       onChange={(e) => {
         typed.current = e.target.value;
+        setTypedOne(isDimensionlessUnit(e.target.value));
         onChange(e.target.value);
       }}
       placeholder={none ? t("labs.unit.none") : placeholder}
@@ -95,15 +99,16 @@ export function LabUnitInput({
         <Checkbox
           id={noneId}
           checked={none}
-          onCheckedChange={(state) =>
+          onCheckedChange={(state) => {
+            setTypedOne(false);
             onChange(
               state === true
                 ? DIMENSIONLESS_UNIT
                 : isDimensionlessUnit(typed.current)
                   ? ""
                   : typed.current,
-            )
-          }
+            );
+          }}
           aria-describedby={hintId}
           className="mt-0.5"
         />