In the aggregate of the seventeen published series of the ABO blood groups of newborn babies and their mothers (an aggregate totalling 53,679 mother-baby combinations) there are substantial reciprocal differences by maternal ABO blood group in respect of the ratio of male to female babies. The ratio is relatively low for AB babies of AB mothers plus A babies of A mothers, but is relatively high for non-AB babies of AB mothers plus non-A babies of A mothers. By contrast, the ratio is relatively high for O babies of O mothers plus B babies of B mothers, but (except in the aggregate of seven of the seventeen series, totalling 16,601 cases) is relatively low for non-O babies of O mothers plus non-B babies of B mothers. Disregarding the babies' blood groups, the sex-ratio is higher for higher for babies of AB than of non-AB mothers. Disregarding the mothers' blood groups, the sex ratio is lower for A than non-A babies, while in the author's own series, included above, the ratio is lower for A babies possessing than for those not possessing detectable A1 antigen. It is suggested that a possible cause of these differences is sex-differential fetal mortality caused by interaction of the ABO genes, and some of the sex-determining genes, with oestrogen and progesterone.