The in vivo and in vitro humoral and cell-mediated immune responses of lymphocytes of BALB/c mouse exposed to mercury chloride$(HgCl_2)$ were investigated. In vitro exposure of the splenocytes to mercury chloride produced overt cytotoxicity in 3 hours period. The $IC_{50}$(the concentration required to inhibit a splenocyte viability by 50%) for mercury chloride was >0.1mM for cytotoxicity. In vivo mercury chloride exposed mice were significantly depressed delayed type hypersensitivity(DTH) response to sheep red blood cells(SRBC) in a dose-dependent manner compared with control group. Mercury chloride inhibited the proliferative responses of splenocytes to lipopolysaccharide, pokeweed mitogen, concanavalin A and phytohemagglutinin in a dose-dependent manner. Hemagglutinin response to SRBC in mercury chloride exposed mice was significantly depressed in a dose-dependent manner compared with control group. After 7 weeks of mercury chloride exposure in vivo, mercury chloride induced an increase of nonspecific serum $IgG_1$ and IgE levels in BALB/c mice.