Proteaae C is a comparatively new proteolytic enzyme obtained as a by-product in the manufacture of streptomycin. In this first study we have found it to be of great value in detecting incomplea anti-Rh and other antibodies present in the rera of prenatal cases. Five per cent saline mapena i m of selected cella were exposed to a 0.1 per cent mlution of protease C, and results were compared with thooe obtained by d n g trypsinizcd and Rdnizcd cells. In general, the results compare favorably with those ubtained with ficin, and this new enzyme is superior to trypoin in titration rtudies. It is predicted that protease G will be a lwful addition to the proteolytic enzymes that are currently used in many blood banking procedures. SINCE 1957 when Morton and Pickles7 first advocated the use of trypsin in the detection of incomplete anti-Rh antibodies, the value of trypsin itself,s. 9.17 and subsequently of papain,3. 5 ficin,l. 2 and bromelin has become well established in Rh antibody screening tests. Although Pirofsky and Mangumla* 13 suggesi that bronielin, in particular, should be employed as a standard crossmatch procedure, it is generally accepted that no one enzyme technic is adequate in itself,6 and most workers employ the indirect Coombs test in all major crossmatches, with the addition of a separate bovine albumin or enzyme method at 37 C.