Organotin compds. have been widely used as stabilizers and anti-fouling agents with the result that they are ubiquitously distributed in the environment. Organotins accumulate in the food chain and potential effects on human health are disquieting. It is not known as yet whether cell surface adsorption or accumulation within the cell, or indeed both is a prerequisite for the toxicity of organotin compds. In this study, the alkylated tin derivs. monomethyltin trichloride (MMT), dimethyltin dichloride (DMT), trimethyltin chloride (TMT) and tetramethyltin (TetraMT) were investigated for cyto- and genotoxic effects in CHO-9 cells in relation to the cellular uptake. To identify genotoxic effects, induction of micronuclei (MN), chromosome aberrations (CA) and sister chromatid exchanges (SCE) were analyzed and the nuclear division index (NDI) was calcd. The cellular uptake was assessed using ICP-MS anal. The toxicity of the tin compds. was also evaluated after forced uptake by electroporation. Our results show that uptake of the organotin compds. was generally low but dose-dependent. Only weak genotoxic effects were obsd. after exposure of cells to DMT and TMT. MMT and TetraMT were neg. in the test systems. After forced uptake by electroporation MMT, DMT and TMT induced significant DNA damage at non-cytotoxic concns. The results presented here indicate a considerable toxicol. potential of some organotin species but demonstrate clearly that the toxicity is modulated by the cellular uptake capability.