HL-A phenotypes were determined in 24 unrelated patients with gluten-sensitive enteropathy (GSE) using a lymphocyte microcytotoxicity test. 21 of the 24 patients had HL-A8 in the second segregant series, a frequency of 0.875. In contrast, the HL-A8 frequency in 200 normal individuals was 0.215 (difference significant at P < 0.002), and in 6 patients with villous atrophy due to tropical sprue or hypogammaglobulinemia the HL-A8 frequency was 0.17 (difference from normal not significant). The HL-A types in the families of three HL-A8 positive patients with GSE indicated that the HL-A8 antigen was inherited as an autosomal dominant. Frequencies of the other HL-A antigens in the GSE group did not differ significantly from that of the normal group. These findings are compatible with the hypothesis that GSE is due to the presence of an abnormal “immune response (Ir) gene,” leading to the production of pathogenic antigluten antibody or, alternatively, to the presence of a particular membrane configuration leading to the binding of gluten to epithelial cells with subsequent tissue damage.