Dextromethorphan hydrobromide (DXM) has been widely used as a nonopioid antitussive drug with low toxicity and low potential for drug dependency. DXM is a psychotropic drug since 2003 in our country. This study was performed to investigate the immunotoxicity induced by abuse of DXM. Mice were orally exposed to DXM dissolved in saline as concentration of 30, 60, and 120 mg/kg b.w. before (day-2) or after (day+2) immunization (OVA-antigen, day 0). Thereafter, we measured the increased rate of body weight, relative weight of organ (thymus, spleen, liver, kidney) and OVA-specific IgM level in sera. In addition, mouse splenocytes were exposed to various concentration of DXM $(0.001{sim}100{mu}M)$ and cultured with B cell mitogen (LPS) and splenocytes proliferations (SP) were measured by MTT-assay. Thymus-weight were slightly changed on day 9 after administration of DXM, but body-, spleen-, liver-, and kidney-weight were not different between control group and DXM-treated group. SP to LPS were significantly decreased at high concentration $(100{mu}M)$ when compared with controls. When DXM was administered before or after immunization with OVA-antigen, OVA-specific IgM levels were significantly lowered in a dose-dependent manner. These results indicate that DXM nay depress the primary humoral immune response to the initial antigenic challenge.