Abstract Lymphocyte proliferation responses were studied in workers with moderately increased body burdens of 2,3,7,8-tetrachlorodibenzo- p-dioxin and other polychlorinated dibenzo- p-dioxins and dibenzofurans (PCDDs/PCDFs, calculated as International Toxicity Equivalencies [I-TE]). Mitogens (pokeweed mitogen [PWM], phytohemagglutinine [PHA], concanavalin A [Con A]), as well as an anti-human monoclonal antibody against CD3 were used as proliferation stimulators in vitro. Additionally, the feasibility of using the lymphocyte response to tetanus toxoid was assessed, and the response to this recall-antigen was included in this trial. No decrease in the capacity of 3H-thymidine incorporation was observed with any of the proliferation stimulators in the group of volunteers with the increased TCDD-body burden when compared with volunteers exhibiting TCDD-concentrations in blood fat within the reference range. Regression analysis revealed a slight trend towards an increase for 3H-thymidine incorporation during the stimulation with PHA only. It can be concluded from our data that moderate increases in the TCDD- or I-TE-body burdens do not induce any medically significant changes in the capacity for proliferation of lymphocytes, measured as 3H-thymidine incorporation.