[6] After clinical training at Mount Sinai Hospital, in New York City, and active service as captain in the U.S. Army Medical Corps, he did a research fellowship in the Department of Biochemistry at NYU.

Weissmann next worked at the Strangeways Research Laboratory, in Cambridge, England, studying cell biology under Dame Honor B.

Starting in 1970, he spent summers as an investigator and lecturer and served for 18 years as a trustee (later emeritus) of the Marine Biological Laboratory Woods Hole, MA.

[7] He was best known for having presented evidence that rheumatoid arthritis is an immune-complex disease (provoked perhaps by genetic programs that misdirect immune responses to oral bacteria).

[8][9] His laboratory found that crises in systemic lupus erythematosus are provoked by intravascular complement activation.

He is a foreign member of the Accademia Nazionale dei Lincei of Rome and the Royal Society of Medicine of London.

His work has been collected in eleven volumes, among them The Woods Hole Cantata (1985) and The Fevers of Reason (2018).

His work was praised for scientific insight by Jonas Salk, for literary style by Kurt Vonnegut, and for breadth of general culture by Adam Gopnik.