A Brazilian family material comprising 468 adults and 839 children has been studied with regard to the inherited serum alkaline phos-phatase variation described by Arfors, Beckman and Lundin (1963). All individuals with an extra serum alkaline phosphatase zone (Pp2) were secretors of ABH blood group substances. There was no correlation with secretion of Lewis substances. The frequency of Pp2 was about 30% in individuals of blood groups B and 0, 15% in the AB-group and only 2% in the A-group. Family studies show that the Pp2 phenotype is dependent on at least 1 additional factor besides the ABO and secretor genes.