To estimate the cellular immune response of workers highly exposed to mixtures of organic solvents, subpopulations of peripheral blood lymphocytes (PBLs) were analyzed. For this, the PBLs of nine floorers (aged 25-58 years, exposure time 8-35 years) were subsequently labelled with monoclonal antibodies OKT4, OKT8, OKT11, anti-Leu 7 and anti-Leu 12. Analysis was made by a FACS IV cell sorter (Becton-Dickinson, USA). The control group consisted of matched pairs of healthy donors. In the exposed group we found a decrease in the OKT11 (all) T cell fraction, a decrease in the OKT4 helper cells, an increase in the anti-Leu 7 positive cells, mostly natural killer cells, an important increase in anti-Leu 12 labelled T cells, i.e., human B-lymphocytes, and no differences in the OKT8 suppressor cells. Total fluorescence intensity profiles between the exposed and the unexposed group did not differ, i.e., the marker density on the cell surfaces remained unchanged. Similar changes in lymphocyte subpopulations are found in states of immunodeficiency and immunogenetic forms of aplastic anemia, a disease whose etiological relationship may be due to long-term exposure to organic solvents.