The human teratocarcinoma cell line Tera 2 can be induced to differentiate in vitro after exposure to retinoic acid. We show in this paper that whereas the K-FGF oncogene is expressed in undifferentiated cells, addition of retinoic acid rapidly (less than 60 min) downregulates the expression of this gene. However, when cells are cultured in RA for an extended period of time (greater than 15 days) K-FGF transcripts reappear. We also report that K-FGF is expressed in approximately one-third of primary human germ cell tumours but not in the corresponding normal testicular tissue.