Lymphocyte cultures from individuals exposed to high levels of hydroarsenicism showed a slower cell cycle kinetics than cultures from low-exposed individuals. Since this difference in proliferation could be due to chronic arsenic exposure, the in vitro effects of inorganic arsenic in human whole blood lymphocyte cultures were investigated. When lymphocytes were exposed to concentrations of arsenite and arsenate similar to those found in the blood of exposed subjects (10(-7), 10(-8) and 10(-9) M) during the last 24 h before harvesting, a dose-related inhibition of proliferation was observed. Cultures were also treated with 10(-9) M of arsenite and arsenate for 2, 6 and 24 h at the beginning of the cultures in the presence or absence of phytohemagglutinin (PHA). Inhibition of stimulation and proliferation was directly related to the length of treatment. The results show that, at the concentrations tested, arsenite and arsenate impair lymphocyte stimulation and proliferation and confirm the fact that chronic arsenic exposure can affect the proliferation of whole blood lymphocytes.