AbstractOral administration of diazepam at doses of 5–10 mg/kg to restraint-stressed mice resulted in almost complete recovery in the stress-induced suppression of the antibody response to sheep red blood cell (SRBC). Noreover, this compound restored the suppression of antibody response to SRBC in cyclophosphamide-treated mice. Diazepam treatment also enhanced the antibody response against SRBC in normal mice only when the animals were immunized with the reduced amount of antigen. It was demonstrated that antigen specific helper T cell activity was promoted by diazepam administration in mice. Addition of diazepam augmented the in vitro anti-SRBC hemolytic plaque-forming cell (PFC) response in mouse splenocytes without altering kinetics of the response. However, the enhancing effect was observed only when the drug was added to the medium at the culture initiation. On the other hand, antibody response to T cell-independent antigens such as trinftrophenylated (TNP)-Ficoll and TNP-lipopolysaccharide were not enhanced by diazepam. Concanavalin A or LPS-induced3H-thymidine uptake into splenocytes were not stimulated by diazepam. These results suggest that diazepam promotes the antibody response through stimulating helper T cell functions.