Antibodies against methylcholanthrene (MCA)-induced sarcomas in Fischer (F344) rats were detected by a modified micro mixed hemadsorption (MHA)-assay. The assays detected anti-tumor antibodies as titers up to 1 : 320 in sera from hyperimmunized rats and at titers up to 1 : 160 in sera from rats bearing a 5-8 cm3 progressively growing tumor. MHA-titers decreased when sera were absorbed with sarcoma cells prior to MHA-assays. IgG antibodies in sera from tumor bearing rats showed titers of 1:20 and 1:5. These anti-tumor sera formed rosettes on the corresponding sarcoma cells as well as other sarcomes induced by MCA in F344 and Lewis strain rats tested. The assay was modified for a micro technique using a microtest plate (No. 3034, Falcon, CA). This modification yielded as assay requiring only 10 microliter of test sera. The test is quantitative and highly sensitive and results are reproducible. Several critical factors which influence test results in this assay were examined.