2,3,7,8-Tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) exposure may be detected by chemical analysis or by its biological effects on in vivo or in vitro systems. Chemical analysis is expensive and does not give indications of the bioavailability of the material to an organism. Dosing an in vitro system with an extract obtained from an environmental sample has numerous advantages. This method is relatively cheap, quite sensitive, and can be used to generate quantitative dose response relationships as well as quantitative structure activity relationships. One method that is currently being used in our laboratories and others is a whole embryo model which consists of fertile hen's eggs injected on day 0. The eggs are incubated for either 48 or 96 hours and the embryos removed and examined for malformations. In addition, alterations in the vitelline vasculature are examined in the 96 hour embryos. This model is able to detect significant 2,3,7,8-TCDD effects at levels of 2–20 pg/g egg (ppt), depending on the endpoint. The results of this study were compared to results of the H4IIE assay based on EROD induction. Although the H4IIE assay is more sensitive, the egg injection method provides more data on effects.