The subjects of investigation included 34 workers at the Phenol Division of the Mazovian Refining and Petrochemical Plants and 18 persons working in administrative positions; the purpose was to elucidate some functions of the immune system cells. The lymphocyte subpopulations (CD3, CD4, CD8) and NK cells were evaluated using monoclonal antibodies and the immunofluorescence method. To search for the functions of lymphocytes and monocytes such as the in vitro production of interleukin-1 (Il-1) and interleukin-2 (Il-2), macrophage inhibitory factor generation (MIF), a cytotoxicity test and T cell proliferation in autologous mixed lymphocyte reaction (AMLR) were also examined. We have shown that workers from the Phenol Division exhibited pronounced deviations in the tested parameters as compared to the administration workers. We found a decrease in CD3 lymphocytes and in Il-1 production. After analysis of the individual results we selected three persons from the Phenol Division who showed abnormal values in four or more parameters (decreased CD4:CD8 ratio, abnormal values of lymphocyte subpopulations, impaired lymphocyte functions in functional tests). These cellular-immunity control tests are promising methods for studies of the biological effects of environmental and/or occupational pollution to toxic derivates of petroleum.