The yolks of White Leghorn chicken (Gallus domesticus) eggs were injected prior to incubation with either 3,3',4,4',5-pentachlorobiphenyl (PCB 126) at doses ranging from 0.1 to 12.8 microg/kg egg or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) at doses ranging from 0.04 to 0.64 microg/kg egg. Chicks were subjected to necropsy within 24 h of hatching. The brain, bursa, heart, liver, and spleen were removed and weighed. Assessment of the rate of hatching indicated an LD50+/-S.E. of 2.3+/-0.19 microg/kg egg (7. 1+/-0.58 nmol/kg egg) for PCB 126 and 0.15 +/- 0.012 microg/kg egg (0.47 +/- 0.037 nmol/kg egg) for TCDD. No significant differences in the incidence of developmental abnormalities (structural defects and edema) were observed in TCDD-exposed embryos, while PCB 126 caused significantly more developmental abnormalities at 3.2, 6.4, and 12.8 microg/kg egg than the vehicle control. PCB 126 caused lower hatchling weights and greater relative brain, heart, and liver weights when compared to the vehicle control group at a dose of 3.2 microg/kg egg which is greater than the LD50. TCDD at 0.08 microg/kg egg caused relative bursa weights to be less than those of the vehicle control. A toxic equivalency factor (TEF) of 0.07 was determined for PCB 126 in relation to TCDD based on overt lethality.