Reverse transcription-PCR and Northern and Western blot analyses indicate that mRNA and protein encoded by the Brachyury gene are expressed by the pluripotent human embryonal carcinoma cell line NTERA2 and are only modestly down-regulated during retinoic acid-induced differentiation. This differentiation occurs along a neural lineage, with no obvious evidence of the formation of mesodermal derivatives. Several other human embryonal carcinoma cell lines that do not differentiate, a yolk sac carcinoma cell line and two choriocarcinoma cell lines, also express readily detectable levels of Brachyury mRNA and protein. Thus, in human teratocarcinomas, Brachyury expression is not necessarily an indicator of commitment to mesodermal differentiation.