diff --git a/api/mac-checker.js b/api/mac-checker.js
index 73a6e17d..057e11ca 100644
--- a/api/mac-checker.js
+++ b/api/mac-checker.js
@@ -7,7 +7,8 @@
 import { normalizeMacQuery } from '../common/mac-input.js';
 import { lookupMac } from '../common/oui-db.js';
 
-export default async (req, res) => {
+// `lookup` is injectable so tests can run the handler over fixture registries.
+export const createMacChecker = (lookup) => async (req, res) => {
     const { mac } = req.query;
     if (!mac) {
         return res.status(400).json({ error: 'No MAC address provided' });
@@ -18,9 +19,11 @@ export default async (req, res) => {
     }
 
     // Past the boot window (requireOfflineData) only a failed download leaves this empty.
-    const result = lookupMac(hex);
+    const result = lookup(hex);
     if (!result) {
         return res.status(503).json({ error: 'MAC database unavailable' });
     }
     res.json(result);
 };
+
+export default createMacChecker(lookupMac);
diff --git a/tests/api-handlers.test.js b/tests/api-handlers.test.js
index 928161e3..d5b7645d 100644
--- a/tests/api-handlers.test.js
+++ b/tests/api-handlers.test.js
@@ -19,12 +19,12 @@ import getUserInfoHandler from '../api/get-user-info.js';
 import getWhoisHandler from '../api/get-whois.js';
 import cfRadarHandler from '../api/cf-radar.js';
 import invisibilityHandler from '../api/invisibility-test.js';
-import macCheckerHandler from '../api/mac-checker.js';
+import macCheckerHandler, { createMacChecker } from '../api/mac-checker.js';
 import githubStarsHandler from '../api/github-stars.js';
 import personaEvaluateHandler from '../api/persona.js';
 import asnProfileHandler, { peeringdbReadiness, connectivityReadiness } from '../api/asn-profile.js';
 import { isAsRelLoaded } from '../common/as-rel-db.js';
-import { isOuiLoaded } from '../common/oui-db.js';
+import { describeMac } from '../common/oui-db.js';
 import { isCompleteProfile } from '../common/asn-profile.js';
 import updateAchievementHandler from '../api/update-user-achievement.js';
 import ipcheckIngHandler from '../api/ipcheck-ing.js';
@@ -595,19 +595,26 @@ describe('mac-checker handler', () => {
         assert.equal(res.statusCode, 400);
     });
 
-    // Reads the repo's local registries: answered when they're on disk,
-    // 503 when this checkout never downloaded them.
-    it('answers a vendor prefix from the local registries, or 503 without them', async () => {
+    // Over fixture registries, so the answer doesn't depend on a download.
+    const fixtureBlocks = new Map([
+        ['3C5AB4', { registry: 'MA-L', company: 'Google, Inc.', address: 'Mountain View US 94043', country: 'US' }],
+    ]);
+
+    it('answers a vendor prefix in any notation from the registries', async () => {
+        const handler = createMacChecker((hex) => describeMac(fixtureBlocks, hex));
         const res = createResponse();
-        await macCheckerHandler(createRequest({ query: { mac: '3c:5a:b4' } }), res);
-        if (!isOuiLoaded()) {
-            assert.equal(res.statusCode, 503);
-            assert.deepEqual(res.body, { error: 'MAC database unavailable' });
-            return;
-        }
+        await handler(createRequest({ query: { mac: '3c5a.b4' } }), res);
         assert.equal(res.statusCode, 200);
-        assert.equal(res.body.success, true);
-        assert.equal(res.body.macPrefix.slice(0, 8), '3C:5A:B4');
+        assert.equal(res.body.found, true);
+        assert.equal(res.body.company, 'Google, Inc.');
+        assert.equal(res.body.blockSize, 16777216);
+    });
+
+    it('answers 503 when no registries are loaded', async () => {
+        const res = createResponse();
+        await createMacChecker(() => null)(createRequest({ query: { mac: '3C:5A:B4' } }), res);
+        assert.equal(res.statusCode, 503);
+        assert.deepEqual(res.body, { error: 'MAC database unavailable' });
     });
 });