Soluble HLA-A,-B antigens have previously been detected in human plasma. More recently, these molecules have been demonstrated to be secreted in water soluble form by cell lines and peripheral blood lymphocytes in vitro due to RNA splicing events which delete exon five from a fraction of class I antigen transcripts. In order to determine whether plasma HLA-A,-B molecules arise by this mechanism, their biochemical properties are analyzed. Based upon molecular weight, detergent binding, and lipid binding properties, it was demonstrated that two forms of HLA-A,-B molecules are present in the plasma. A 43 kd form that binds both lipids and detergents is presumed to be a shed version of the membrane form. However, a 39 kd form that fails to bind lipids and detergents has properties identical to those molecules secreted in vitro. It is suggested that these molecules arise by alternative splicing events in vivo that are identical to those characterized in vitro. Although all tested individuals display shed and/or secreted plasma class I molecules, it is striking that HLA-A24 occurs in both a shed and a secreted form at levels higher than all other alleles examined.