We examined the in vivo effect of 2-amino-4,4 alpha-dihydro-4 alpha,7- dimethyl-3H-phenoxazine-3-one (Phx) on Meth A carcinoma cells transplanted into BALB/c mice, in terms of both antitumor activity and side effects. Phx, which was synthesized by the reaction of 2-amino-5-methylphenol with bovine hemolysates, was administered i.p. at doses of 1 and 5 mg/kg to BALB/c mice transplanted with Meth A tumor cells. Phx exerted a strong antitumor activity to Meth A tumor growing in the mice as 5-fluorouracil (5-FU) did. The antitumor activity of Phx at the dose of 5 mg/kg was comparable to that of 5-FU at the dose of 7.8 mg/kg. In contrast, unlike 5-FU, Phx did not cause leukopenia while showing a strong antitumor activity. The compound also produced little changes in body weight and no wasting of mice developed. These results show that Phx has strong anti-tumor activity, but exerts lower side effects and suggest that Phx is available for therapeutic purposes in the future. [(C) 2000 Lippincott Williams & Wilkins.].