Bovine immunoglobulin G subclass (IgG1 and IgG2) antibodies were found to fix bovine complement while only IgG1 fixed guinea-pig complement in vitro. Similar results were noted when IgG1 and IgG2 antibodies were tested by passive cutaneous anaphylaxis (PCA) in that both IgG1 and IgG2 caused PCA in bovine skin while only IgG1 mediated the reaction in rat skin. In precipitation reactions IgG1 antibodies to DNP failed to cause precipitation of DNP19-ovalbumin while IgG2 antibodies to DNP precipitated DNP19-ovalbumin. Both IgG1 and IgG2 antibodies to ovalbumin precipitated ovalbumin. Surprisingly, IgG2 antibodies to equine erythrocytes caused phagocytosis by bovine neutrophils and peripheral blood monocytes while IgG1 antibodies failed to cause either phagocytosis or adherence. Results with peripheral blood monocytes cultured for 7 days demonstrated that both IgG1 and IgG2 could mediate phagocytosis.