The effects of dithiocarbamates on the mitotic activity of thymocytes were studied in vitro and in vivo. The mitotic activity was determined by counting of the metaphases blocked by colchicine. Using thymocytes of mice the mitotic activity in short-time cultures was inhibited (50%) by the following concentrations of the substances under study (EC50-values in mol/l): tetraethylthiuramdisulfide; (1; disulfiram) 4.2.10(-9); diethyldithiocarbamate (2) 3.2.10(-8), Cu-2, 2.6.10(-8), Zn-dimethyldithiocarbamate (3; Ziram) 1.9.10(-8), Zn-ethylene-bis-dithiocarbamate (4; Zineb) 2.3.10(-6). Using rat and guinea-pig thymocytes the ED50- values for 1 and 2 were found in the same concentration range as compared with thymocytes of mice. There was no impact on the vitality of cells by the mitosis inhibiting concentrations. In vivo the mitotic activity of thymocytes in mice was decreased only after application of high doses (500 mg/kg i.p.) of 2. After depletion of reduced glutathione (GSH) by the pretreatment of mice with diethylmaleate the effects of 2 were increased in vivo. In vitro GSH (10(-4) mol/l) decreased the mitosis inhibiting activity of 2 in mice thymocytes and the inhibition of the SH-enzyme ecto-ATPase in rat thymocytes. It is supposed that the inhibition of mitosis is due to the reaction of dithiocarbamates with SH-groups of microtubules and other functionally important proteines. The formation of tiyl radicals from dithiocarbamates may play a certain role in redox processes which can be influenced by GSH.