Ludwik Gross (September 11, 1904 – July 19, 1999) was a Polish-American virologist who discovered two different tumor viruses—murine leukemia virus and mouse polyomavirus—capable of causing cancers in laboratory mice.

After the war, he joined other scientists (notably Rosalyn Yalow, recipient of the 1977 Nobel Prize in Medicine or Physiology) in the "Golden Age" of research at the Bronx Veterans Administration Medical Center, becoming director of the Cancer Research Division.

One story claims that this appointment allowed him to move his research mice from the trunk of his car, where he had been carrying out studies, into a fully equipped laboratory.

[citation needed] He died at Montefiore Medical Center on July 19, 1999, of stomach cancer at age 94.

In the course of these studies, he isolated the Gross murine leukemia virus strain, as well as the first polyomavirus (so named for its proclivity to cause cancers in multiple tissue types).