The Wnt gene family encodes a series of conserved glycoproteins that regulate pattern formation during embryogenesis, in a variety of tissues including the nervous system. As with other genes that control embryonic cell differentiation, members of the Wnt family have also been implicated in tumourigenesis. To search for Wnt genes involved in human teratocarcinomas, with a possible role in human embryogenesis, we used RT-PCR primed with degenerate oligonucleotides to analyse mRNA from differentiating cultures of the pluripotent human embryonal carcinoma (EC) cell line NTERA-2. NTERA-2 EC cells differentiate into neurons and other cell types when induced with retinoic acid. Wnt gene expression was not detected in the undifferentiated EC cells, but Wnt-related PCR fragments were amplified from differentiating cultures, 4-14 days after induction with retinoic acid. The RT-PCR products were composed primarily of DNA fragments corresponding to the recently identified human Wnt-13 gene. No other Wnt-related genes were identified. Northern analysis confirmed induction of Wnt-13 as a 2.4 kb mRNA during the early phases of retinoic acid-induced differentiation, and during differentiation along a non-neural pathway induced by hexamethylene bisacetamide (HMBA), but not in the terminally differentiated neurons. Wnt-13 remained expressed in non-neural differentiated NTERA-2 cells, even several weeks after the induction of differentiation. The time course of induction, its induction by HMBA, and its persistence in differentiated cells indicate that Wnt-13 expression is not dependent upon direct activation by retinoic acid. Wnt-13 was not detected, or only detected at low levels, in other human EC cells. However, it was found to be expressed at a high level in one malignant teratoma cell line, 577MF, that does not exhibit an EC phenotype although it was derived from a testicular teratocarcinoma. At least two members of the human frizzled gene family, thought to encode receptors for Wnt proteins, were also expressed in the NTERA-2 cells, suggesting the presence of a mechanism by which endogenously expressed Wnt-13 could modulate the histogenesis of teratocarcinomas by mediating interactions between sub-populations of differentiating EC cells. We note that Wnt-13 maps to chromosome 1p13, a region reported to be subject to relatively frequent loss of heterozygosity in germ cell tumours. Further analysis indicated that 465 bp of the published Wnt-13 sequence, within the predicted 5' UTR, is incorrect and is possibly derived from a human mitochondrial DNA sequence.