Actinomycin D injected simultaneously with sheep erythrocytes in female rats caused a delay in the immune response but had no effect on the rate or maximum amount of hemagglutinin produced. The delay was roughly proportional to the concentration of the antibiotic administered, and was up to 2 days for 75 microg in a 200-gram female rat (sublethal dose for females). The dose effect in the delay in response is consistent with the time when actinomycin would no longer be available to bind with newly synthesized DNA and when messenger-RNA production could occur. Similar results were obtained with another antigen, the enzyme beta-galactosidase, in male rats during the secondary response.