Secondary sex ratio (SR) in man is influenced by various genetic and environmental factors. It has been observed that SR in subjects of blood group B compatible with their mothers is higher than in other subjects. The analysis of 676 newborns of the Rome population and 1,684 newborns of the New Haven (Connecticut) population have confirmed a higher SR in B group subjects compatible with their mothers. The data also indicate that placental alkaline phosphatase is another genetic factor influencing SR in man and that there is a strong interaction among ABO phenotype, fetomaternal ABO compatible status and PAP phenotype concerning their effects on SR.