In 108 workers occupationally exposed to organic solvents containing benzene, toluene and xylene, the activity of acid phosphatase (AP) was determined in the peripheral blood lymphocytes. A positive correlation was found between the number of lymphocytes exhibiting redistribution of AP from lysosomes into the cytoplasm and the exposure time to the solvents. The cytochemical estimation of the lymphocyte AP activity might practically be used as a screening test for the early detection of the toxic effects of exposure to the above solvents.