SUMMARY This paper presents basic data concerning a heterophile system shown in previous studies to be related to human renal transplantation. Patients with either acute infections from some gram-negative bacteria or acute renal allograft rejections developed substantially higher titers of human antirat erythrocyte antibody than is common in normal people. The heterophile antibodies produced as a consequence of either infection or acute renal allograft rejection reacted with antigens present on rat erythrocytes but not on sheep erythrocytes. Absorption and neutralization studies confirmed the presence of antigens in or on human kidneys, various gram-negative bacteria, and rat erythrocytes which will react in this heterophile system. The antigens are similar but are not serologically identical to the common antigen (CA) of Enterobacteriaceae. Some patients in chronic renal failure were shown to be immunized to the heterophile antigens, presumably from previous infection.