HLA-A2, -A9, and -A10 antigen titers in serum were assessed by the lymphocytotoxicity inhibition test on family bloods of neonates (cord bloods) and parents in order to investigate their genetic control. (1) Serum HLA-A9 and -A10 antigens were distributed among parents with higher titers than neonates, while serum HLA-A2 antigen was found with almost equal titers among parents and neonates. (2) Correlation of serum HLA antigen titers was calculated between parents and neonates: A2 and A9 antigen titers showed positive correlations. AIM antigen was hardly detectable in sera of neonates. (3) Neonates whose parents were weak inhibitors had low titers of that antigen, while neonates of strong inhibitor parents had high or low titers. (4) Discrepancy was observed among the HLA antigen titers of different specificities in heterozygous individuals. It was difficult to group individuals into secretors and nonsecretors of whole HLA antigens.