Detailed 1977 national natality data are used to investigate social, demographic, and genetic effects on birthweight differentials. Analysis of birthweight differences among infants with white, black, and mixed black-white parents indicates that a portion of the observed weight differentials appear to be due to biologic factors. Infants with a black mother and father have the lowest mean birthweights, while infants with two white parents have the highest weights. Newborns with mixed-race parents have intermediate birthweight distributions. Multivariate analysis suggests that the effects of parental race on birthweight are not the result of maternal/obstetric differences among parents of the same or mixed race.