June 1, 1674

Antonie van Leeuwenhoek

Leeuwenhoek reports microscopic observations of blood and human tissues

The best-documented date sits a year earlier than the calendar entry: by 1 June 1674, the supposed beginning had already happened.

A letter from Delft, 1 June 1674

On 1 June 1674 Antoni van Leeuwenhoek sent Henry Oldenburg a six-page letter describing microscopic observations of blood, skin, bone, teeth, liver, brain, spinal marrow, and muscular tissue, together with techniques for examining them.

No consulted source identifies a discrete moment in 1675 when he began examining human tissues and liquids. The 1674 letter shows the work was already under way; it does not date his first private experiment.

Three packets

The letter arrived with drawings of capillary tubes and an envelope holding specimens of cork, elder pith, and the white material from a quill.

When those original packets were examined in 1981, more than 70 percent of the material was still present.

In print, and what came next

An extract appeared in the 1674 Philosophical Transactions under a title listing blood, milk, bones, brain, spit, and cuticle, occupying pages 121 to 128.

Leeuwenhoek did make a different discovery around mid-September 1675 — living creatures in stored rainwater — but he gave that date retrospectively, in his letter of 9 October 1676.

His first observation of bacteria belongs to 1676.

Sources

Researched 24 Aug 2026 5 sources not yet audited Date disputed

How this was checked

Researched from the web into a fact sheet and rewritten from that sheet. This article has not yet had an independent model audit. How the pipeline works →

Date disputed. Sources disagree; the research places it at 1674-06-01.

What the sources leave uncertain

No consulted scholarly or primary source identifies a discrete 1675 moment when Leeuwenhoek began examining human tissues and liquids.
The strongest documentary match is the letter of 1 June 1674. It proves that such observations were already underway, but it does not establish the date of Leeuwenhoek’s first private experiment.
Leeuwenhoek did make a different discovery around mid-September 1675—living creatures in stored rainwater—but he reported that date retrospectively in his letter of 9 October 1676.