July 27, 1921

Frederick Banting

Banting and Best's first promising pancreatic-extract test in a Toronto dog

The experimental subject has no name in the surviving records — only the number 410. Its blood sugar fell within the hour; by the next morning it was comatose.

A crude extract, a numbered dog, and one hour of readings

Nothing was announced on 27 July 1921. The date is a widely repeated anniversary for work in J. J. R. Macleod's University of Toronto physiology laboratory, where Frederick Banting — a physician and surgeon, not a biochemist — and the student Charles Best were injecting extracts of degenerated dog pancreas into dogs whose own pancreases had been removed.

After early surgical failures, the pair had two dogs whose pancreatic ducts had been tied for about seven weeks. They removed the degenerated pancreas of Dog 391 and prepared an extract from it.

What they produced was a crude preparation they called "isletin," not chemically pure insulin.

27 July or 30 July?

The University of Toronto's archive-based exhibition puts the preparation and injection on the morning of 30 July, into a mildly diabetic terrier recorded only as Dog 410. Banting and Best's readings showed its blood sugar falling from 0.20 percent to 0.12 percent in the hour after the injection.

The commemorated 27 July appears to derive from the surviving Dog 410 chart, which is dated across 27–31 July. The archive-based reconstruction places the first injection and promising result on 30 July.

The experiments themselves had begun on 17 May 1921, according to that same archive.

What the result was worth, and what it was not

The remaining experiments that day were inconclusive. Dog 410 was comatose the next morning and died later that day, and no autopsy was performed.

The numbers cannot now be checked: the pair worked with a crude colorimeter and may not have sampled quickly enough for the readings to be trusted. They wrote excitedly to Macleod, who was away at the time, but they had neither purified a human-ready drug nor announced a discovery.

J. B. Collip joined the work in December 1921 and produced extract pure enough for clinical use, and the first widely publicized announcement came when Macleod presented the Toronto group's clinical evidence on 3 May 1922.

Sources

Researched 23 Aug 2026 8 sources 2 audit passes Date disputed

How this was checked

Researched from the web into a fact sheet, rewritten from that sheet, then audited against it by a different model. How the pipeline works →

Date disputed. Sources disagree; the research places it at ~1921-07-27.

What the sources leave uncertain

27 July is widely repeated in commemorative secondary accounts as the date insulin was isolated. The University of Toronto's surviving chart is dated 27–31 July, which likely helped generate that tradition.
The archive-based University of Toronto exhibition places preparation and injection of the extract on the morning of 30 July. That is the best-supported date for the actual experimental milestone.
The July material was a crude pancreatic extract called 'isletin,' not isolated chemically pure insulin. Even the accuracy of the blood-sugar readings cannot now be established because the researchers used a crude colorimeter and may not have sampled quickly enough.