Summary: A thermolabile antistaphylococcic opsonin is found in active guinea-pig serum. The opsonin operates as a function of its concentration but no zone-phenomenon is noted. The magnitude of opsonization by the fresh serum is the same whether the organism is viable or not, but the opsonic action of active guinea-pig serum is greater for an avirulent strain of staphylococci than for a virulent strain. The opsonic activity of active guinea-pig serum is optimal in hypotonic solutions and is markedly reduced in hypertonic solutions. Since, however, a staphylococcic rabbit-antiserum operates optimally at a physiological salt solution range, this finding may serve as a means of differentiating between the two sera. The staphylococcic antiserum shows a typical zone-effect and its active principle is relatively thermostable. The active guinea-pig serum and the antiserum require from 20–30 minutes for optimal activity but during the first few minutes active guinea-pig serum shows a greater opsonic effect than the antiserum. The addition of active guinea-pig serum to optimal and suboptimal concentrations of the staphylococcic rabbit-antiserum prevents the latter from exerting its full opsonic activity so that the level of opsonization is found in the range of the effect of the added normal serum. The addition of concentrated antiserum to active guinea-pig serum results in an inhibition of the fresh serum action and the prozone-effect of the antiserum becomes evident.