Clams--Ruditapes decussata--were transplanted to an organotin-polluted marina for a period up to 5 weeks. Tributyltin (TBT) was the major organotin compound detected in clams; TBT accumulation was extremely rapid, it reached a maximum tissue concentration of 290 ng/g wet weight 3 weeks after transplant, which represents a bioaccumulation factor of 12,000. Dibutyltin, monobutyltin and triphenyltin were also detected, although at a lesser extent (4-22 ng/g wet wt). Investigations on endogenous steroid content by radioimmunoassay revealed important changes in TBT-exposed clams. A 33% increase in testosterone titres was observed 5 weeks after transplant. Likewise, a time-dependent decrease in oestradiol levels was evidenced, viz. oestradiol titres decreased 2, 3.5 and 5-fold in clams sampled 1, 3 and 5 weeks after transplant, respectively. These observations together with previous data on TBT-exposure experiments suggest a potential masculinization of clam physiology as a consequence of TBT exposure.