Soluble HLA antigens present in normal human serum

HLA antigens have been detected in normal human serum by inhibition of monospecific HLA antisera in the complement-dependent cytotoxicity test. The antigens present in serum were the same as those found on the donor's lymphocytes. HLA-A9 was unique in that it was present in serum at inhibitory titers of 1:8 to 1:32 whereas the majority of A-and B-locus antigens were present at titers of 1:4 or less. Serum HLA-A9 antigens were found in the high density lipoprotein fraction with a molecular weight greater than 200,000. In the presence of detergent they have an apparent molecular weight of 86,000 which is reduced to 46,000 when treated with papain. These values are similar to those obtained for detergent-solubilized cellular HLA antigens.