Human antimalignin antibody (AMA) appears to have clinical significance because in actuarial studies its concentration relates quantitatively to survival (Bogoch et al. Protides Biol Fluids 1984; 31:739-747). Therefore isolation, characterization, and production in vitro of AMA were undertaken. Serum AMA concentrations are elevated in cancer, regardless of cell type, as demonstrated by earlier blind studies of 1,026 (Bogoch et al. J. Med 1982; 13:49-69) and 501 (Bogoch and Bogoch. Protides Biol Fluids 1983; 30:337-352) and independently confirmed by others on 354 (Bogoch et al. Protides Biol Fluids 1984; 31: 739-747) cancer patients and controls. Mouse monoclonal AMA was produced earlier (Bogoch et al. Lancet 1981; 2:141-142). To validate the identity of the natural substrate AMA in the serum determination (AMAS test) and to prepare for human imaging and therapeutic trials, human AMA has now been produced in vitro from human lymphocytes and has been shown to be increased when primed with its specific 10,000-dalton peptide antigen malignin. This synthesized human AMA adsorbs specifically to its immobilized antigen in vitro and resembles in cancer cell staining and in other properties human AMA isolated from sera of cancer patients and mouse monoclonal AMA. All are predominantly IgM, as shown by reduction to heavy and light chains followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis.