Suppression of the primary response of rabbits to intravenously administered KLH can be achieved with very small amounts of hyperimmune anti-KLH administered a day later since the rabbit apparently rapidly eliminates most of the KLH by nonimmunologic means. The amount of passive anti-KLH needed to achieve immunosuppression was directly proportional to the dose of injected antigen. Antibody passively administered as much as 6-8 days after antigen still can be strongly immunosuppressive, which suggests that the antibody must be reacting with immunogen in or on responding cells or perhaps in the process of transfer between cells. There was no evidence that the presence of passively administered hyperimmune anti-KLH prior to the injection of antigen had any immunosuppressive action beyond the direct neutralization of the injected antigen. When KLH was injected in Freund adjuvant, anti-KLH incorporated with the KLH in the adjuvant was much more efficient in causing immunosuppression than anti-KLH given intravenously. The primary responses to 2 mg KLH given intravenously and 2 microg given in adjuvant reached approximately equal peaks and were suppressible by comparable amounts of intravenously administered anti-KLH. Two observations suggest that passive antibody neutralizes the immunogenic stimulus at the level of individual antigenic determinants and not merely by aggregating or precipitating entire antigenic molecules. First, anti-abalone hemocyanin (AH) which cross-reacts approximately 50% with KLH was only partially immunosuppressive even in extremely large amounts, i.e., amounts which could react with and precipitate much more KLH than could the smaller but more suppressive doses of anti-KLH. Second, when KLH and anti-KLH were given together in adjuvant, effective immunosuppression was achieved only with amounts of anti-KLH sufficient to saturate or cover virtually all available antigenic determinants. The immunosuppressive quality of passive antibody increases with time after immunization and with repeated immunization of the donor. In view of their relatively weak immunosuppressive properties, antibodies formed in the first weeks of a primary response may not contribute significantly to the turning off of the antibody response. In any event, results obtained by passive transfer of hyperimmune antibody to animals early in a primary response cannot be applied to the natural events in a primary response.