A series of genetic, developmental and environmental variables have been analyzed in a prospective sample of full-term newborn babies, compatible with their mothers in the major blood group systems, in order to attempt an evaluation of the effect of these variables on serum bilirubin level during the first few days of life. Three genetic factors (PGM1, ACP1 and ADA) and three non-genetic variables (rise of bilirubin level during the first day of life, a mother with a history of previous abortion, and use of alcoholic beverages by the mother) have a significant predictive value for the separation of newborns with clinically relevant jaundice from other infants.