In utero exposure to environmental dioxin-like, estrogen and androgen compounds can cause adverse health effects. Little is known about potential interactions in vivo between dioxin-like compounds, estrogens and androgens during fetal development in humans. Therefore we explored the potential interactions in vivo between dioxin-like compounds, estrogens, androgens using chemical-activated luciferase expression (CALUX)(R) bioassays in maternal and umbilical cord blood plasma concurrently collected at the time of planned Caesarean section from 98 healthy pregnancies. The dioxin-like activity was also determined after placental transfer of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in the ex vivo human placenta perfusion system. Similar dioxin-like activity in maternal and cord blood (37 versus 33pg CALUX(R)-TEQ/g plasma lipids, P>0.05) was detected and it demonstrates transplacental transfer. Increased dioxin-like activity in the perfused placenta tissue after ex vivo TCDD perfusions (from 17 to 280pg CALUX(R)-TEQ/g plasma lipids) suggest that accumulation in the placenta prevents immediate transplacental transfer of TCDD. Androgenic activity were also similar in the paired mother-newborns (0.10 versus 0.18ng CALUX(R)-AEQ/mL plasma), whereas cord blood plasma estrogenic activity was higher than maternal levels (22.6 versus 18.5ng CALUX(R)-EEQ/mL plasma). In cord blood plasma androgenic activity was strongly positively associated with maternal levels (Rs=0.8, P<0.001) whereas dioxin-like and estrogenic activities were modestly associated with maternal levels (Rs</=0.4, P<0.001). The micronuclei frequency, an indicator of genetic instability was significantly associated with dioxin-like activity in cord blood, independently of other recorded factors (Rs=0.4, P<0.003). This study demonstrated interactions in vivo between dioxin-like, estrogenic and androgenic exposures during fetal development of humans.