Sera of 154 recipients of renal allografts were studied for transplantation heterophile (T-H) antibodies by means of immunodiffusion, mixed agglutination (MA) and enzyme immunoassay (EIA). T-H antibodies were found by immunodiffusion against bovine red blood cell (BRBC) extracts (15%) and sheep red blood cell (SRBC) extracts (12%): The specificity of antibodies to BRBC was shown to be distinct from that of antibodies to SRBC. Both of these T-H antibody types were absorbable by guinea pig kidney ( GPK ) tissue sediments and, therefore, they could be classified into the GPK -positive group of heterophile antibodies. The MA test was successfully employed to demonstrate directly T-H antibodies combining with antigens of GPK . Results of the MA inhibition studies and those of EIA indicated that some of the BRBC antibodies are directed to antigens of asialo-high molecular-weight glycoprotein of BRBC.