The detection of antibodies to H-2 and species cell-surface antigens, on suspensions of leukemia and MC-induced sarcomas by an assay using fluorescein-labelled protein A from Staphylococcus aureus is described. The presence of ‘foreign’ H-2 allospeci-ficities on MC-induced sarcoma cells were detected. Antibodies to leukemia and MC sarcoma cells showed a high degree of cross-reactivity in their reaction with different MC-induced sarcomas and with leukemia cells.