We tested the hypothesis that abnormalities of central immune function are genetically controlled in patients with systemic lupus erythematosus. We used an in vitro suppressor-cell assay to evaluate central immunoregulation in 15 patients, 50 of their clinically healthy family members and 41 normal persons. Impaired suppressor-cell function was found in 11 patients; there was no correlation between disease activity and test results. Abnormal suppressor-cell activity was also found in 13 first-degree relatives, 12 of whom were women. We found no correlation between results of the suppressor-cell assay and the presence or absence of lymphocytotoxic antibodies in the relatives. Impaired suppressor-cell function cannot by itself explain the pathogenesis of systemic lupus erythematosus. Our results support the hypothesis that certain abnormalities of suppressor cells are genetic markers. We propose that the development of systemic lupus erythematosus requires the participation of at least two functionally distinct classes of genes.