Concentrations of diethylstilboestrol (DES) exceeding 100 microM are normally fatal to all living tissues due to inhibition of cell division and other cell processes. However, in Caenorhabditis elegans, with increasing levels of DES, there was an observed decrease in fertility and fecundity (99%), although a small percentage of the population (1%) survived concentrations as high as 400 microM DES. Uptake studies, using tritiated-DES liquid scintillation and ELISA analysis, revealed that uptake exhibited saturation at 100 microM and levels exceeding 100 microM resulted in no further biological response. Thus, ultrastructural morphology and viability of the organism was unchanged after the DES saturation point.