The human erythrocyte complement receptor type 1 (CR1) is polymorphic with respect to molecular weight. Size variants with molecular weights of 190,000 (type A), 220,000 (type B) and 160,000 (type C) daltons have been detected in normal individuals (22 individuals), patients with hydralazine (Hz) lupus (n = 27), a group of Hz controls (n = 30) and the relatives of both Hz groups (27 and 11 individuals, respectively). The method of detection was SDS-polyacrylamide gel electrophoresis of erythrocyte membranes on low-percentage cross-linked gels followed by Western blotting using polyclonal rabbit anti-CR1 antibodies. In normal individuals, 77% had the A allotype and 26% carried the B allotype; amongst Hz lupus patients 67% carried the A allotype, 31% carried the B allotype, and 3% (1 individual) had the C allotype. Amongst the patients who had been on Hz but did not develop SLE, 83% carried the A allotype and 17% carried the B allotype. The AA phenotype was only found in 44% of Hz SLE patients but in 64% of normals and 70% of the Hz control group. Although not statistically significant, the results indicate a relative underrepresentation of the AA phenotype in patients with Hz-induced SLE. In addition, an equal or greater relative amount of the C allotype was detected in an Hz SLE patient with the AC phenotype. This is in contrast to lower relative amounts of the C allotype found in normal individuals with this phenotype.