A total of 203 couples with unexplained habitual abortions and 364 consecutive normal puerperae along with their live-born babies were studied. The analysis of wife-husband joint ABO blood group distribution in couples with habitual abortion showed an excess of A incompatible mating type and a defect of B incompatible type as compared with expected proportions assuming random mating. The joint wife-husband ABO blood group distribution was further analysed in relation to the adenosine deaminase (ADA) genotype. A defect of O-A and A-O couples when the wife carries the ADA*1/*1 genotype and the husband carries the ADA*2 allele, and a defect of O-O and A-A when the wife carries the ADA*2 allele were observed. In the sample of normal puerperae, analysis of the joint mother-newborn ABO distribution in relation to the ADA genotype showed a pattern similar to that observed in couples with habitual abortion, i.e. there is a defect of O-A and A-O when the mother carries the ADA*1/*1 genotype and the newborn carries the ADA*2 allele and a defect of O-O and A-A types when the mother carries the ADA*2 allele. Altogether the data suggest an early loss of O-A and A-O zygotes when they carry the ADA*2 allele and an early loss of O-O and A-A zygotes when the mother carries the ADA*2 allele resulting in a deficit of these zygotic classes among both spontaneously aborted fetuses and live-born infants. The pattern of association observed in the mother-fetus type O-A (incompatible according to conventional terminology) appears similar to that observed for the reciprocal A-O type (compatible according to conventional terminology). Therefore strictly conventional immunological mechanisms cannot explain the whole pattern of associations. Cell to cell intereactions involving ABO antigens may have an important role at implantation: ADA, through the control of local adenosine concentration, could modulate these interactions influencing the probability of successful implantation.