By the histochemical semi-quantitative method, the activity of nonspecific EN esterase in lymphocytes of those occupationally exposed to benzene, toluene and xylene was determined. The airborne concentrations at the workstations were: benzene--0-370, toluene--0-580, xylene--0-560 mg/m3. A reduced number of lymphocytes containing EN in lysosomes and increased number of cells exhibiting EN activity in lysosomes and cytoplasm were found in workers employed over 54 months. That would imply a damage to lysosomes with subsequent displacement of lysosomal enzymes to cytoplasm. In view of the positive correlation between exposure duration and number of lysosomes with damaged EN--positive lysosomes, the authors suggest the histochemical method of semi-quantitative determination of EN activity may be applied to evaluate toxic effects of benzene and its homologues upon lymphocytes.