The prevalence of atopic diseases appears to have increased rapidly, especially in industrialized countries. The increase may be explained by a change in certain environmental factors. This article focuses on the influence of environmental factors on IgE production. Epidemiological or experimental reports have shown that tobacco smoke, virus infection and mercuric chloride may enhance IgE production. We demonstrated the enhancing effect of diesel-exhaust particulates (DEP), which seem to have increased in urban environments, on IgE antibody production. The IgE antibody responses in mice immunized by intraperitoneal injection of antigens mixed with DEP were higher than those in animals immunized with the antigens alone. DEP also had an adjuvant activity for IgE antibody production in mice after entry via the respiratory tract (the natural mode of entry). The enhancing effect of DEP on IgE antibody responses was demonstrated even when a small dose such as 1 micrograms of DEP was given intranasally at three-week intervals. Our further study has indicated that suspended particulate matter including materials other than DEP has an adjuvant activity for IgE antibody production.