Antisera to different morphologic classes of human leukemia cells, produced in monkeys and chimpanzees, were, after appropriate absorptions, cytotoxic to leukocytes from patients with leukemia and some myeloproliferative disorders but not to leukocytes from normal donors or patients with other neoplastic and non-neoplastic diseases. Direct cytotoxicity testing and absorption studies demonstrated that nonhuman primate antisera to cells from 2 different donors with chronic lymphocytic leukemia probably detected an antigen common to all acute and chronic lymphocytic leukemia patients. The nonhuman primate antisera to myeloid leukemia cells, however, detected more than one leukemia-associated antigen. Absorption studies on the nonhuman primate antisera to myeloid leukemia ceUs indicated an antigen specific for certain acute myelogenous leukemia cells and another antigen specific for certain chronic granulocytic leukemia cells. Another antigen(s) was cross reactive between cells from these 2 types of myeloid leukemia. Antisera to subcellular antigen components from human leukemia cells, produced in rabbits, were, after absorptions, cytotoxic to cells from leukemia patients but not to leukocytes from normal donors. The rabbit antisera, however, did not distinguish between the morphologic classes of leukemia. Nonhuman primate antisera to monkey myelogenous leukemia cells and a tissue culture cell line derived from a monkey myelogenous leukemia also detected neoantigens present on some human leukemic cells. The spectrum of antigens present on the surface of leukemic cells may all be considered leukemia-associated antigens.