Summary: A new method of passive immune lysis is presented. This method involves the coupling of protein antigen to chromic chloride-treated human erythrocytes. The kinetics of CrCl3 requirements, antigen dosage, and incubation are presented. The antigen-coated erythrocytes are lysed by an excess of rabbit complement. Experiments documenting the sensitivity and reproducibility of the test as well as the stability and fragility of CrCl3-treated erythrocytes are presented, and various advantages of the test are discussed.