AbstractThe effect of benzo(a)pyrene (BaP) at different molar (MI concentrations on the in vitro anti-sheep red blood cell (SRBC) plaque (antibody) forming cell (PFCI response and the one-way mixed lymphocyte response (MLR) was tested. Inhibition of the PFC response and the MLR occurred when spleen cells were exposed to a wide range of BaP concentrations from 10–4m to 10–8m. Maximum depression of the responses occurred at 10–5M for PFC production (47% of controls) and for the MLR (19% of controls) as measured by a stimulation index. No significant loss in cell viability was observed at this or lower molar concentrations of BaP. The non-carcinogenic analog of BaP, benzo(e)pyrene, did not suppress PFC responses at comparable concentrations. This in vitro system will facilitate manipulations of T and B lymphocytes and macrophages (adherent cells) in a controlled culture environment for precisely characterizing the sensitivity of these cells and their subpopu-lations on exposure to BaP.