Summary Polyriboinosinic–polyribocytidilic acid stimulated formation of hemolytic antibodies in mice immunized with sheep erythrocytes. Similarly, poly Ir:Cr treatment of mice resulted in a marked reduction in survival of isografts. Temporarily, administration of poly Ir:Cr at the time of exposure of mice to antigen resulted in optimal enhancement of humoral and cellular antibody formation. Optimal enhancement of both humoral and cellular antibody formation was induced by 100-200 μg of poly Ir:Cr, while significant stimulation of hemolytic antibody formation was induced by a lower dose of 10 μg of poly Ir:Cr. We wish to thank Mr. Hugh Pettigrew of the Biometry Branch, National Cancer Institute, for the statistical evaluation of the experimental data.