The effect of deltamethrin (10- or 30-day exposure) on selected parameters of humoral and cellular immune response in mice was studied. In parallel also hematologic and histologic examinations were performed. It was found that deltamethrin exerts negligible immunotropic effect. The prevailing was suppressive effect, and in some cases also stimulatory effect (PFC, GvH, Il-1, exogenous CFU-s). This negligible activity on the immunological system may be attributed to low toxicity of deltamehtrin in relation to the cells of immunological system or to too short exposure of the animals to its effect.