Nude, putatively athymic mice injected with a single high dose of sheep erythrocytes fail to respond by producing splenic plaque‐forming cells when injected 1 week later with T cells plus sheep erythrocytes. Spleen cells from such mice are defective in their capacity to transfer at least an IgG plaque‐forming cell response to sheep erythrocytes when injected into irradiated recipients together with T cells. The results suggest that T‐cell‐independent antibody production in nude mice is shortly followed by a defect in B‐cell responsiveness. A recent hypothesis, which ascribes a protective function of T cells for B cells of high antigen‐binding capacity, is furthe