Our family studies and the mother-child studies show that there is a maternal effect. Jana Hesser (then a graduate student in Anthropology working in our laboratory) and Ioanna Economidou, Stephanos Hadziyannis, and our other Greek colleagues collected information on the sex of the offspring of parents in a Greek province of Macedonia. In this community the probability of infection with HBV is very high and a majority of the parents had evidence of infection, i.e. detectable HBsAg and/or anti-HBs in their blood. It was found that if either parent was a carrier of HBsAg there were significantly more male offspring than in other matings (38). In subsequent studies using the Greek data and additional data from Mali in West Africa, London, Drew, and Veronique Barrois (a post-doctoral trainee from Paris) have found that there is a deficiency of male offspring when parents have anti-HBs (39). This had led London and his colleagues to test the hypothesis that anti-HBs has specificities in common with H-Y or other antigens determined by genes on the Y chromosome. If these observations are supported by additional studies, then HBV may have a significant effect on the composition of populations in places where it is common, which includes the most populous regions of the world. The ratio of males and females in a population has a profound effect on population size as well as on the sociology of the population. This connection of HBV with sex selection may also explain why there is a greater likelihood of rejection of male kidneys by renal patients with anti-HBs, and indicate how kidneys can be better selected for transplantation. Transplantation of organs and pregnancy have certain immunologic features in common. Rejection of male kidneys and "rejection" of the male fetus may be mediated by similar biological effects.