beta 2-glycoprotein I (beta 2I) is a 50kDa serum glycoprotein of ill defined function. Based upon its capacity to bind negatively charged phospholipids a number of possible inhibitory roles for beta 2I have been proposed. We have cloned and sequenced a full length mouse beta 2I cDNA clone and demonstrated that mouse beta 2I does not behave as an acute phase reactant following an experimentally induced inflammation. Phylogenetic analysis of the known mammalian beta 2I homologues has provided evidence that mouse beta 2I is the most divergent and is evolving at a faster rate than beta 2I in other species.