A reconstituted water medium was developed for use in the Frog Embryo Teratogenesis Assay--Xenopus (FETAX). FETAX solution was then tested on three compounds with known mammalian teratogenicity (ethanol, caffeine, and 5-fluorouracil) as was a non-teratogen (saccharin). The results obtained were then compared with results from tests on these compounds in two other media that had previously been used in the assay. Saccharin was not teratogenic. Ethanol and caffeine were weak and moderate teratogens, respectively. 5-fluorouracil was a strong teratogen. The results compare favorably with those obtained in mammalian studies. The amount of growth inhibition in embryos in the 96 h tests was positively correlated with the degree of teratogenicity of the compound. Final validation of FETAX will allow it to be used to screen and rank compounds for further testing and as a tool for studying the basic mechanisms of teratogenesis.