The in vitro and in vivo effects of the newly synthesized compound TOK-8801 (N-(2-phenylethyl)-3,6,6-trimethyl-5,6-dihydroimidazo[2,1-b] thiazole-2-carboxamide) on immune responses were investigated in comparison with that of levamisole (LMS). TOK-8801 enhanced the anti-sheep red blood cells (SRBC) plaque forming cells (PFC) response and mitogen-induced proliferative responses in murine splenocytes and thymocytes in vitro at concentrations of 10(-7)-10(-5) M, while LMS augmented these responses at 10(-4) M. The stimulatory effect of TOK-8801 as well as LMS on the antibody production was eliminated by the removal of T cells. TOK-8801 (0.5 mg/kg, p.o.) suppressed the in vivo generation of anti-SRBC PFC in normal mice, but the compound restored the depressed antibody production in restraint-stress mice which was mainly caused by helper T cell defects. From these results, TOK-8801 was shown to have an immunomodulatory activity in the antibody production.