Adult mice can be depleted of essentially all mature alpha beta T lymphocytes by chronic treatment with the framework-recognizing, pan-specific anti-TCR alpha beta mAb, H57-597. Similar findings have been reported in rats, gamma delta cell populations remain essentially unaltered in size and reactivity. Suppression of alpha beta T-cell development results in the loss of alloantigen reactivity and of B-cell help, suggesting that gamma delta and alpha beta populations differ in their functional capabilities. Indirect effects of the antibody treatment include quantitative changes in splenic B cells, as well as reduced sizes and weights of experimental animals. alpha beta-suppressed mice and rats may provide model systems for studies on gamma delta cell function in vivo.