At the National Institute of Chemical Safety we have surveyed the immunological status of donors from the oil industry, health services, and metallurgy exposed to different immunotoxic compounds. Their data were compared to those of healthy, non-exposed controls. Our aim was to study the relationship between immunotoxic exposure and immune function, and to establish a system of immunological parameters by which chemical exposure can be specifically monitored. Subpopulations and activation of lymphocytes was measured by flow cytometry, using immunophenotyping of peripheral blood lymphocytes. In the groups exposed to immunotoxic compounds we found an increase in helper, and a decrease in cytotoxic T lymphocytes, leading to a shift in Th/Tc ratios. These phenomena are not substance specific, but relate to chemical exposure. The lymphocytes of exposed groups showed a higher proportion of activated cells, but there was a difference in the expressed activation markers. Our results suggest that characterizing lymphocyte subpopulations and activation markers on PBL of donors is a useful tool in tracking environmental immunotoxic effects.