Protease G 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 incomplete anti‐Rh and other antibodies present in the sera of prenatal cases. Five per cent saline suspensions of selected cells were exposed to a 0.1 per cent solution of protease G, and results were compared with those obtained by using trypsinized and ficinized cells. In general, the results compare favorably with those obtained with ficin, and this new enzyme is superior to trypsin in titration studies. It is predicted that protease G will be a useful addition to the proteolytic enzymes that are currently used in many blood banking procedu