In non-smoking policemen from a town of Central Italy, blood CD4+ lymphocytes were reduced and CD8+ were increased as compared with a control group. This immunological alteration was not evident in the smoking policemen. Urine lead (marker of exposure to toxic agents produced by traffic) and blood natural killer (NK) CD16+ lymphocytes as well as serum copper and HLA-DR+ cells (B, T, NK activated lymphocytes and monocytes) were significantly correlated in the whole group of 42 examined subjects. Another study was performed on 15 healthy men, occupationally not exposed to toxic agents and living in a suburban area. Their urine lead, was positively correlated with the serum IgA immunoglobulins and negatively correlated with blood CD5(+)-CD19+ (a B subset bearing the T CD5 antigen) lymphocytes. On the contrary, urine chromium was negatively correlated with serum IgA and positively correlated with CD16(+)-56+ NK and CD5(+)-CD19+ B lymphocytes as well as with HLA-DR+cells. Serum zinc was also correlated with total HLA-DR+and CD3-HLA+DR+ (activated B and NK lymphocytes and monocytes) cells. These later data suggest that only zinc and copper but also trivalent chromium (to which normal population is mainly exposed in ordinary environmental conditions) may play a role in the mechanisms regulating the immune response.