Serum protein markers (alpha 1-AT, Bf, C3, C4A, C4B, Hp and Tf) were studied in a series of 36 patients with systemic lupus erythematosus (SLE) and compared to normal blood donors. In agreement with the results of previous investigations a significant increase of complement C4 deficiency was found among the SLE patients. The relative risks for AQ0 and BQ0 homozygosity were 7.2 and 4.1, respectively. Simultaneous occurrence of AQ0 and BQ0 was found in three patients with a calculated relative risk of about 65. A significant increase of the haptoglobin type 2-2 (p less than 0.05) was found among SLE patients. The remaining serum protein systems showed no statistically significant associations with SLE.