A method for detection of the primary binding of soluble tumor-associated antigens by antibodies has been developed by using an enzyme immunoassay (EIA). A heteroantiserum was produced by injecting tumor cells from a chemically induced murine sarcoma into rabbits, and antibodies reacting with most normal tissue components were removed by exhaustive in vivo absorption. A soluble preparation of tumor cells, obtained by 3 M KCl extraction, was conjugated to beta-galactosidase from Escherichia coli. The antibody binding was measured by determining the enzyme activity that could be separated by anti-antibody coprecepitation. The reaction follows saturation kinetics, and nonlabeled antigen can be readily quantitated by inhibition. The present method detects determinants common to several MC-induced tumors on the same mouse strain but absent in normal cells and nonrelated tumors in addition to individual tumor-specific transplantation antigens. The sensitivity and simplicity of the new method compare favorably with a binding assay that utilizes radioactive iodine as a label. Thus, EIA becomes a flexible tool for the further characterization and purification of these antigens.