Summary: Cortisone has been shown to inhibit the primary response in rabbits, as measured by antigen elimination techniques, to a small dose of a soluble antigen. The data suggest that this inhibitory effect is probably exerted early in the sequence of events leading to antibody synthesis. Despite variation in time of initiation and duration of cortisone treatment, inhibition of the secondary response occurred in only 50% of the animals. The ability of bacterial endotoxins to incite antibody formation in cortisone-inhibited rabbits was established.