Several molecular weight forms of plasminogen activator (PA) activity have been observed in serum-free conditioned media from human cells in culture. An antibody inhibition technique is described which combines inhibition of enzyme activity by anti-urokinase IgG with sodium dodecyl sulfate-gel electrophoresis to determine whether different molecular weight forms of human cell PA's are immunologically related to urokinase. Plasminogen activator forms with molecular weights of 85,000 to 95,000, 50,000 to 60,000, and 36,000 were inhibited by anti-urokinase IgG. In contrast, PA forms with molecular weights in the 73,000 range from three different types of human cells were not inhibited by comparable concentrations of the antibody. Human embryonic kidney cultures contain only anti-urokinase IgG-inhibitable PA forms, while melanoma-derived Malme-3M cultures contain only anti-urokinase IgG-resistant forms. Cultures of tumorigenic Detroit 562 cells and nontumorigenic IMR-90 cells contain a mixture of "antibody-sensitive" and "antibody-resistant" PA forms. The antibody-resistant 73,000-dalton PA form may be a precursor of the smaller antibody-sensitive, urokinase-related forms, or it may be the product of a second plasminogen activator gene which codes for a protein immunologically and structurally different from urokinase.