The aim of this study was to find a suitable biomarker for pyrethroid adverse effects. It was shown that there is a correlation between the half-life time (t(1/2)) of pyrethroids in plasma and the clinical findings. We hypothized that this finding indicates an interindividual different amount of total esterase activity or even a polymorphism. By in vitro experiments it was demonstrated that pyrethroids are cleaved by carboxylesterases. After it turned out that carboxylesterase activity in human plasma is too low for detection, a method for specific determination of carboxylesterase activity in human isolated lymphocytes was developed. As a substrate for carboxylesterase activity, cyfluthrin was added to the lymphocyte suspension. As a proof for cyfluthrin degradation by carboxylesterases the produced hydrocyanic acid was determined by GC/MS. First hints for interindividual differences in carboxylesterase activity in lymphocytes were found.