OBJECTIVE To evaluate the in vivo radical scavenger activity of vitamin E, vitamin C, and beta carotene on erythrocyte membranes. METHODS A prospective, open trial without placebo. METHODS Department of Clinical Pharmacy. METHODS Ten healthy volunteers being supplemented with beta carotene, vitamin E, and vitamin C. METHODS Erythrocytes were incubated in water bath with 2,2' azobis (2 amidinopropane) hydrochloride (AAPH). AAPH decomposes spontaneously at 37 degrees C to generate free radicals inducing membrane cellular damage and hemolysis. The absorbance was measured at 405 nm at 0, 30, and 60 min, and then every 20 minutes for four hours. The time for 50 percent of maximal hemolysis (T50%), which expresses the radical scavenger activity of erythrocytes, was determined. RESULTS The physiologic T50% value determined in 52 healthy volunteers is 117 +/- 12 min. Patients receiving these supplements have a higher value of T50% (143.2 +/- 11.6 min at 30 d and 145.7 +/- 10.5 min at 60 d) than the physiologic value (p < 0.001). CONCLUSIONS These data suggest that vitamin C, vitamin E, and beta carotene stimulate the radical scavenger activity of erythrocyte membranes after 30 days.