Studies carried out on inbred strains of mice have shown that conceptuses which differ at the MHC antigens from their mothers appear to enjoy a selective advantage when compared with conceptuses which are more compatible. In humans a highly significant degree of MHC compatibility can be found in couples with a history of repetitive spontaneous abortions with unknown aetiology. We HLA - typed 28 selected couples with a history of three or more consecutive spontaneous abortions of unknown aetiology and 28 normal couples as control. We found that 22/23 (79%) aborter couples shared common HLA antigens, while normally fertile couples only 7/28 (25%) (p less than 0.001). The finding of a significant HLA compatibility in couples having abortions might be consistent with the hypothesis that blocking antibodies, formed in early pregnancy as response to HLA antigens, are perhaps necessary for a successful gestation. The factor causing abortion in couples sharing HLA antigens might also refer to the homozygosity for fetal genes in linkage with HLA alleles. The sharing of HLA alleles could be a marker for other genes of the same region which are lethal for the embryo in the homozygous state.