SUMMARYRabbits bearing skin allografts were treated with methyl-prednisolone to delay graft rejection. The recipients were studied serologically for histocompatibility antibodies by the antiglobulin consumption test, mixed agglutination, lymphoagglutination, and hemagglutination. No serum antibodies were detected in the drug treated host by the latter three methods. Data obtained from antiglobulin consumption show that the usual lag phase between rejection of a graft and the appearance of serum antibodies which is so characteristic of unmodified rejection is greatly reduced if not eliminated in rejection delayed by cortisone. This raises the possibility of a serologic method of diagnosing rejection.