Human β2 microglobulin has been tested for 3 of the known effector functions of human immunoglobulin G, namely complement activation, opsonization and sensitization of mast cells. The ability of β microglobulin to fix complement was demonstrated by a complement fixation assay following aggregation of the protein by adsorption on latex or chemical cross linking with bisdiazobenzidine (BDB) and also by a CI inhibition assay using the native protein. In the latter assay β microglobulin was as effective as human Fc in binding CI and showed a similar dose response curve. When coated on sheep erythrocytes β microglobulin was able to promote rosette formation by guinea pig peritoneal macrophages. These properties have previously been shown to be associated with different homology regions of the Fc fragment. β microglobulin was not able to participate in PCA or RPCA reactions in the guinea pig.