Abstract The immunopharmacological effects of a newly synthesized compound in vitro, TOK-8801 ( N-(2-phenylethyl)-3,6,6-dihydroimidazol[2,1-b]thiazole-2-carboxamine), on the anti-SRBC plaque-forming cell (PFC) response and delayed-type hypersensitivity (DTH) reaction were investigated. Oral administration of TOK-8801 (0.1–10 mg/kg) resulted in the suppression of the PFC responses to varying doses of antigen (5×10 6, 2×10 7, 1×10 8) in C3H/HE strain mice (7 W) which are high responders to SRBC antigen. On the other hand, the compound augmented the PFC response in aged mice (8–9 months) in which the PFC response was markedly depressed compared with that in young mice. In the experiment of the methylated human serum albumin-induced DTH reaction, TOK-8801 augmented the reaction,in low responder (C57BL/6) mice by oral administration of 0.1–1 mg/kg for 5 days from the sensitization, whereas suppressed the reaction in high responder (ICR) mice. These immunopharmacological actions of TOK-8801 were compared in dose and activity with those of lobenzarit and bucillamine. Thus, these results suggest that TOK-8801 may act as an immunomodulating agent and would be expected to be a useful agent for autoimmune diseases such as rheumatoid arthritis.