We have studied the antibody response in vitro of spleen cells from C57BL/6 mice kept on a protein deficient (D) or a normal diet (N). Short or long term protein restriction initiated after weaning led to increased plaque forming cell (PFC) responses to sheep red blood cells (SRBC), TNP-ficoll and TNP lipopolysaccharide. The influence of dietary restriction on the suppression of the antibody response to SRBC was studied in mixed cultures of antigen sensitized and fresh, non-immune cells from either D or N donors. Addition of pre-sensitized D or N cells to non-immune N spleen cells in a 1:1000 ratio resulted in marked suppression of the PFC response whereas co-cultures of pre-sensitized cells and non-immune D spleen cells did not result in significant suppression. Similarly, non-immune T cells from DF mice exerted a lower suppressor effect than non-immune T cells from N mice. Either dietary restriction or low dose cyclophosphamide treatment of the donors of non-immune spleen cells determined a similar reduction in suppression. It is suggested that nutritional deficiency selectively depletes short-lived suppressor effector lymphocytes which are activated in the presence of antigen-stimulated inducer cells.