An analytical method for the quantitative determination of monobutyltin (MBT), dibutyltin (DBT), tributyltin (TBT) and triphenyltin (TPhT) compounds in human breast milk is described. After the addition of surrogates (deuterium derivatives), milk samples were extracted with hexane-diethyl ether (4:6) in the presence of HCl and NaCl. Each extract was purified by cation exchange chromatography and treated with Grignard reagent to yield ethyl derivatives, which were determined by gas chromatography (GC) with flame photometric detection operated in the tin mode (610 nm). These organotinchlorides, spiked to milk at 12.5, 25, and 50 ng/ml (ppb), were recovered within a range of 85 to 105%. Detection limits were 1.3 ng/ml for DBT, TBT, and TPhT, and 2.5 ng/ml for MBT. This analytical method was used to determine organotins in about 70 breast milk samples obtained from mothers who had given birth within the previous week. DBT dichloride levels varied from undetectable to 9.5 ng/ml in human milk from mothers who habitually ate fish, however, the other organotins were not detectable. No significant difference was observed in DBT contents between mothers who ate fish more than twice a week and those who ate fish less than once a week. Thus, since the levels of organotin even in the milk of mothers who liked to eat fish were very low, human breast milk should be considered safe for feeding infants, at least concerning with regard to the possible transmission of organotin compounds.