Human apolipoprotein H (APOH) is associated with lipoprotein present in plasma. It has been shown that APOH has structural similarities with the regulation of complement activation (RCA) protein superfamily and is involved in phospholipid binding interactions on platelets and as an autoantigen in complex with anionic phospholipids. Nevertheless, additional functional studies are necessary to establish the physiological role of APOH. By hybridizing a cDNA probe for APOH to a panel of somatic cell hybrids, we show that the structural locus for this protein maps to 17q23→qter and is therefore not part of the RCA cluster on chromosome 1. The site of biosynthesis for APOH was established by Northern blot analysis. Hybridization of the APOH cDNA probe to total liver RNA identified a transcript of approximately 1.5 k