ABSTRACT: We have confirmed that many women who suffer from recurrent spontaneous abortions have an increased incidence of histocompatibility (HLA-A, HLA-B, HLA-C, and HLA-DR) antigen-sharing with their husbands. We also found abortion-prone women who did not share HLA with their mates; however, they failed to respond or responded suboptimally to their husbands when compared to third-party controls in the mixed lymphocyte culture (MLC) reactions. Some aborting patients manifested a husband-specific MLC blocking factor in their plasma which was not present in their serum. In contrast, nonpregnant childbearing wives did not share as many of their husbands' HLA antigens as frequently, had normal or heightened MLC responses to their husbands, and showed no evidence of humoral blocking factors. We propose that HLA typing, MLC reactions, and blocking factor studies will serve as useful laboratory indices in future clinical studies of immunogenetical disorders associated with idiopathic habitual abortion.