Serum alpha 2HS-glycoprotein (A2HSG) concentrations of 63 patients with systemic lupus erythematosus (SLE) were determined, and found to be significantly low compared to those of 59 healthy blood donors. The diminution of serum A2HSG concentration was proportional to the degree of activity of SLE, and was not influenced by secondary infections. There was a statistically significant positive correlation between serum A2HSG and the C3 complement component levels. A negative correlation between serum A2HSG and IgG, IgA concentration and anti-DNA activity was observed. Serum A2HSG was significantly low in cases of positive for the following laboratory parameters: anti-nuclear antibodies, circulating immune complexes and LE cell phenomenon. We found no correlation between serum IgM concentration, cryoglobulins, latex agglutination and serum A2HSG levels. The unusually good negative correlation between A2HSG pathogenetical role of this glycoprotein in SLE. The determination of A2HSG concentration may be of clinical importance in SLE.