Plasma total lipid, cholesterol, all-trans retinol (vitamin A) and alpha-tocopherol (vitamin E) concentrations, and the susceptibility of erythrocytes to hydrogen peroxide-induced haemolysis in vitro were investigated in healthy laboratory-bred common marmosets (Callithrix jacchus). The concentrations of alpha-tocopherol and total cholesterol were similar to those of control human subjects. The mean lipid concentration was higher (P less than 0.025) and the retinol concentration lower (P less than 0.001) in the marmosets. The susceptibility of the erythrocytes of the marmosets to hydrogen peroxide-induced haemolysis was high, both in absolute value and relative to the controls. Changes in phospholipid and fatty acid composition might have rendered the red cells susceptible to oxidative stress.