Synthetic pyrethroids are increasingly used as insecticides and are claimed to have a relatively low human toxicity. The aim of this study was to examine the in vitro effects of the synthetic pyrethroid S-bioallethrin alone and in combination with the common synergist piperonyl-butoxide (PBO) on human blood lymphocytes and basophils in atopic individuals and non-atopic control subjects. S-bioallethrin and PBO also caused inhibition of lymphocyte proliferation (MTT-test) after a 72-h culture period in a concentration dependent manner. In contrast to the MTT-measurements the combined agents are more effective in inhibiting interleukin-4 (IL-4)- and interferon-gamma (IFN-gamma)-production. The regulatory IL-4/IFN-gamma balance showed a significant difference between atopic and non-atopic subjects after a culture period of 24-48 h in the presence of micromolar S-bioallethrin (P < 0.001). Furthermore S-bioallethrin, PBO and the combined agents induced histamine release from human basophils. Although this effect was little compared to histamine liberators like FMLP and anti-IgE, the response to S-bioallethrin and PBO was significantly different in atopic donors compared with non-atopics (P < 0.01). In scratch test experiments 4 of 18 tested atopic volunteers showed positive reaction (wheals and flares) to S-bioallethrin and permethrin, whereas no reaction could be measured in the control group (age-matched). These findings demonstrate the immuno- and allergo-toxicological properties of the synthetic pyrethroid S-bioallethrin combined with the synergistic PBO using this in vitro approach with human lymphocytes and basophils.