Antisera to a synthetic c-myc peptide and to c-myc antigens synthesized from various portions of the human gene expressed in Escherichia coli were used in order to characterize the protein product of the human c-myc oncogene. Although the deduced molecular weight of the human c-myc protein is 49,000, these antisera precipitate a protein from human cells that migrates in sodium dodecyl sulfate-polyacrylamide gel as if its molecular weight were 65,000. In addition, the mouse c-myc protein, whether synthesized in cells or in a cell-free system directed by pure, synthetic messenger RNA, has analogous properties and is immunoprecipitated by the antiserum to the human c-myc protein. Similar proteins are immunoprecipitated from monkey, rat, hamster, and frog cells, suggesting evolutionary conservation of antigenic structure of the c-myc protein among vertebrates. In addition, and in a manner consistent with the behavior of its messenger RNA, the immunoprecipitable c-myc protein is sharply induced by the action of mitogens on resting human T cells.