The only in vitro method that has been widely accepted as capable of consistently providing evidence for the presence of type specific protective antibody has been the opsonophagocytic test. The other in vitro methods except the long chain test employ M protein extracts in various states of purification and are hampered by cross reactions to non type specific M associated antigens (NTSM). Variables involved in micro cft for type specific (TSM) and non type specific (NTSM) antibodies against preparations of streptococcal M protein were studied. Sera of rabbits immunized with purified M protein which contained high titers of anti TSM and low titers of anti NTSM antibodies reacted type specifically in the cft when relatively low concentrations of M protein were employed. In contrast to these artificially induced TSM antibody responses, the sera of rheumatic fever patients demonstrated elevated CF antibody titers (1:80 to 1:320) in the presence of both high and low concentrations of M antigen. The cft cannot distinguish TSM and NTSM antibodies when the latter predominate. The cft can be employed with confidence for the detection of NTSM antibodies, provided that the test sera are known to lack type specific antibodies against the M protein used in the test.