Abstract: The in vitro metabolic system comprised NADP co-factors and liver microsomes isolated from male rats pre-treated with phenobarbital, 60 mg/kg, i.p for 3d combined with a single i.p. dose of 80 mg/kg of naphtholflavone. The 1% RBC suspension was made up from G6PD-defitient rabbit blood. Primaquine, chloroquine and M 8506 at various dosages were incubated at 37℃ with the microsomal metabolic system in vitro respectively. The supernatants were incubated with 1% RBC suspension. The OD635nm values of tbe supernatants were detected after incubation and centrifugation. The results showed that both primaquine and M 8506 exhibited potent hemolytic toxicity at the dose-range of 1.5 -3 × (101-103) μmol/L, with certain dose-effect relationship, while chloroquine exhibited no hemolytic toxicity. It is suggested that the in vitro assay incorporated with microsomal metabolic system might be a useful preliminary screening method for testing hemolytic toxicity of various antimalarials.