In the study, the immunosuppressive potential of ME and MAA was evaluated in young adult female Fischer 344 rats and C57BL/6J mice. Rats and mice were dosed by gavage with either ME or MAA in water, at dosages ranging from 50 to 400 mg/kg/d, for 10 consecutive days. Rats and mice were examined for alterations in body, spleen and thymus weights, and mitogen-induced proliferation of splenic lymphocytes in vitro; separate groups were employed for the antibody plaque-forming cell (PFC) response to trinitrophenyl-lipopolysaccharide (TNP-LPS). Rats dosed at 100-400 mg/kg/d ME and rats dosed at 50-400 mg/kg/d MAA had decreased thymus weights in the absence of decreased body or spleen weights. Lymphoproliferative (LP) responses to concanavalin A (Con A), hytohemagglutinin (PHA), pokeweed mitogen (PWM) and Salmonella typhimurium mitogen (STM) were all reduced in rats treated with all dosages of ME and at dosages of MAA at or greater than 100mg/kg/d. In contrast to the effects of ME and MAA on these end points in the rat, no thymic involution or suppression of LP responses were observed in mice dosed at 50-400 mg/kg/d.