Most members of seven families in whom one subject was known to have congenital adrenal hyperplasia due to 21-hydroxylase deficiency were haplotyped for HLA B antigen which was supposed to be linked to the defective gene. Heterozygous males (group 1, n = 9), and females (group 3, n = 8) were recognized. These had no clinical abnormalities. All heterozygotes, as well as eight normal males (group 2) and ten normal females (group 4) received in IM injection of Synacthen. Plasma samples obtained before (PRE) and three hours after (POST) the injection was assayed for cortisol, testosterone, androstenedione, 17-hydroxyprogesterone, and dehydroepiandrosterone. All steroids increased significantly after ACTH except for testosterone. The ratio cortisol/17-OHP (F/17-OHP) decreased in group 1. POST 17-OHP (ng/ml) (x +/- SEM) was higher and F/17-OHP lower in groups 1 and 3 respectively: (5.83 +/- 1.83, 60 +/- 20; 4.53 +/- 1.13, 81 +/- 34) than in groups 2 and 4: (1.95 +/- 0.72, 196 +/- 67; 1.94 +/- 0.65, 269 +/- 167). POST F levels were not different among groups. When the distributions were standardized, individual POST 17-OHP levels of heterozygotes were different from the mean POST 17-OHP of the respective normal group and vice versa. The two normal males from the affected families could also be separated from heterozygous but not from normal males.