We examined the in vitro effects of 2-amino-4, 4α-dihydro-4α, 7-dimethyl-3H-phenoxazine-3-one(Phx 1)and 3-amino-1, 4α-dihydro-4α, 8-dimethyl-2H-phenoxazine-2-one (Phx 2) on the proliferation of phytohemagglutinin (PHA)- or anti-human IgM-activated human peripheral blood mononuclear cells (PBMC). Phx 1 and Phx 2 inhibited the incorporation of ³H-thymidine of PHA-activated PBMC by as much as 75% and 40%, respectively, at a concentration of 40 μM. The inhibition was dependent on the dose of Phx 1 and Phx 2. These results strongly suggest that Phx 1 and Phx 2 inhibit proliferation of T cells, because PHA specifically activates the T cells among PBMC. On the other hand, when PBMC were activated by anti-human IgM, which specifically stimulates B cells, the incorporation of ³H-thymidine was rather increased in the presence of 15.8 μM Phx 1 or Phx 2. However, at a higher concentration of Phx 1 or Phx 2 (50 μM), the incorporation of ³H-thymidine was increased by Phx 1, but was inhibited by Phx 2. These results suggest different effects of Phx 1 and Phx 2 on proliferation of human T and B cells.