He taught at Donaldson Preparatory School, Ilchester, Maryland, for one year before beginning graduate work at the University of Michigan.
His work on the genetics of breast cancer in mice, begun during his graduate years at Michigan, was continued at the newly formed Roscoe B. Jackson Memorial Laboratory at Bar Harbor, Maine.
After a fire at the Jackson Laboratory in 1947, Bittner was one of the main contributors of replacement mice because he still had the purebred mouse strains needed to continue their research.
In other words, nursing mice transmit this agent, or "Bittner virus", in their milk, which ultimately leads to tumors in their offspring.
In addition, Bittner felt that genetic and hormonal influences, along with the milk agent, contributed to the origin of cancer.
In 1966, it was proven that Bittner's "milk factor" was a virus that remained dormant during the early life of the young mouse but produced cancer when hormonal conditions were right in middle age.
"Making Mice Standardizing Animals For American Biomedical Research 1900-1955" Princeton University Press (2004) Time Magazine articles: