Numerous chemicals released into the environment by man are able to disrupt the functioning of the endocrine system by binding to estrogen receptors in estrogen-responsive cells. The ability of o,p'-dichlorodiphenyl trichloroethane (DDT) and methoxychlor to compete with estradiol for binding to estrogen receptors in MCF-7 cells (relative binding affinity; RBA) was examined in both serum-free medium and 100% serum; this is referred to as a relative binding affinity-serum modified access (RBA-SMA) assay. RBA's ranged from 0.04% for o,p'-DDT (which showed enhanced access to cells in serum relative to serum-free medium) to 0.004% for methoxychlor (which did not show enhanced access in serum). Based on these findings, these pesticides, along with diethylstilbestrol (DES) as a positive control, were fed to pregnant mice from days 11-17 of pregnancy. When the male offspring were examined in adulthood for their rate of urine marking in a novel territory (territorial behaviour), the rate of urine marking increased dramatically with low doses of DES (relative to controls) and then decreased significantly at the highest dose administered prenatally. Relative binding in MCF-7 cells accurately predicted the doses of o,p'-DDT and methoxychlor that produced the same results, providing support for the hypothesis that effects on behaviour were mediated by binding to estrogen receptors in the developing brain.