Rabbit antisera to myelogenous leukemia (ML) cells were raised; ML cells from line K-562 that has the Philadelphia (Ph) chromosome were used as antigen. Antibodydependent, complement-mediated cytotoxicity was demonstrated by the trypan blue test and Cr release assay for cultured ML cells, whereas no cytotoxicity was demonstrated for cells from B (SB) and T (MOLT 4) lymphoblastoid cell lines. The antisera showed no cross-reactivity for normal human peripheral leukocytes or purified granulocytes. A low level (less than 8%) of cytotoxicity was directed against cell membrane associated fetal bovine serum proteins. Absorption of the immune serum with normal human bone marrow cells of first trimester human whole embryo cells reduced the cytotoxic titer to a similar extent; this suggested the possibility of crossreactivity between ML cells and fetal antigen(s). However, the ML antigen(s) was unrelated to carcinoembryonic antigen (CEA), since absorption with CEA had no effect on the serum cytotoxic titer. The anti-ML sera were cytotoxic for cells taken from 10 patients with chronic myelogenous leukemia and from 3 with acute myelogenous leukemia. In contrast, the leukocytes of 1 of 4 patients with acute lymphocytic leukemia, and 3 of 7 with chronic lymphocytic leukemia shared similar antigenic determinants as demonstrated by cytotoxicity tests. The significance of the cross-reactivity of some lymphatic and ML cells may be the result of the use of rabbit sera that did not distinguish antigens common to both granulocytic and lymphocytic cells, or it may reflect an "immature" or "blastic" antigen present on many leukemia cells.