The alloantibody response of LEW rats immunized with DA kidney homogenate or given vascularized DA kidney grafts was studied using an 125I-anti-Ig binding assay with kidney homogenate as the target tissue. A major component of the sera was found to be directed at a kidney-specific alloantigen. Analysis of LEW X (DA X LEW) F1 backcross rats showed that the locus coding for this antigen was not linked to the major histocompatibility complex (MHC). The use of these backcross rats, typed for both MHC and the kidney alloantigen, as kidney donors to LEW rats showed that the kidney alloantigen did not induce detectable graft damage and that only the DA MHC was involved in primary graft rejection of DA kidneys by LEW rats. Immunofluorescence studies localized the antigen to the basement membrane of proximal convoluted tubules, and to some Bowman's capsules. Strain distribution studies showed that only 2 of 11 strains were negative for the allele under study; these were the LEW and AS strains. Species cross-reactivity studies showed that rabbit, dog, and human kidneys could absorb this specificity, although more weakly than DA kidney.