SummaryCon A‐pretreated mononuclear (MNC) cells from Thai adults with naturally acquired P. falciparum or P. vivax malaria were significantly less effective in suppressing the responsiveness of autologous or normal allogeneic responder cells to mitogenic lectins or allogenic stimulator cells than pretreated cells from healthy donors. Serial studies of three patients demonstrated that reduced suppressor cell activity was present early in malaria infection but returned to normal soon after treatment. These studies demonstrate that the loss of T cells previously observed in patients with malaria, in part may functionally represent a loss of suppressor T cell