We examined B-lymphocyte alloantigens in 41 patients with systemic lupus erythematosus and 184 controls, using a panel of 47 pregnancy serums, and compared reaction frequencies of individual serums. One serum, la-715, reacted with B lymphocytes from 75.6 per cent of patients and 14.1 per cent of controls (Pc less than 0.005, relative risk 18.8). Twenty-eight of the patients were also typed with a panel of HLA-D-related serums from the Seventh International Histocompatibility Workshop, HLA-DRw types assigned, and compared to 490 Workshop controls. Both HLA-DRw2 (57.1 per cent vs. 26.4 per cent, Pc less than 0.004) and HLA-DRw3 (46.4 per cent vs. 22.2 per cent, Pc less than 0.03) were increased in systemic lupus erythematosus. This study demonstrates that select B-lymphocyte alloantigens, which are controlled by genes in the major histocompatibility complex, are present in increased frequency in systemic lupus erythematosus.