SUMMARY.Papain, bromelin and ficin can be standardized by the casein method for use in red blood cell antibody screening tests. The minimal and optimal enzyme activity for detecting blood group antibodies in donors, using the new Automated Pre‐Transfusion Blood Testing System Olympus PK7200 by the two‐stage method for all three enzymes, is one casein unit. One casein unit of proteinase activity changed the red blood cell surface charge to a low plateau value as measured by electrophoretic mobility and sialic acid content, and removed sterically inhibiting structures of surface protein as detected by SDS