In this report the evaluation of a safety factor is assessed, according to the decision tree suggested by Renwick (1991a,b), to determine a Tolerable Daily Intake (TDI) value for the environmental contaminant bis(tri‐n‐butyltin)oxide (TBTO). Based on available literature the nature of the most sensitive parameter of TBTO‐toxicity was perceived to be on lymphoid organs and lymphoid function. Subsequently, safety factors were derived in relation to published data on inter‐species and inter‐individual differences in both kinetics and dynamics of TBTO.