Soluble HLA class I antigens in human plasma preparations possibly play a role in HLA sensitization and modulation of the immune response. We therefore have determined their concentration in albumin and immunoglobulin preparations from several commercial sources and compared these values to the concentration in normal human sera. For this purpose we used a newly developed solid-phase enzyme immunoassay employing rabbit anti-mouse antibody, monomorphic HLA class I monoclonal antibody and a polyclonal enzyme-linked β(2)-microglobulin-specific antiserum. Soluble HLA antigen concentration in 14 albumin batches from 6 manufacturers and in 16 immunoglobulin batches from 11 manufacturers ranged from 0 to 9.6 and from 0 to 20.9ng/ml. The concentration in normal human sera 1,328 ± 954 ng/ml (n = 54). We conclude that the concentration of soluble HLA concentration in albumin and immunoglobulin preparations is more than 50 times lower than in normal human serum, but considerable differences exist between products of various manufacturers.