A mouse testis cDNA library in lambda pCEV27 eukaryotic expression vector was transfected in NIH3T3 fibroblasts and several transformed foci were identified. A plasmid with high-titered focus forming activity was rescued from one of these transformants. Structural analysis of this cDNA predicted a protein identical to androgen induced growth factor (AIGF), recently identified as the eighth member of the fibroblast growth factor (FGF) family. A 1.6 kilobasepair transcript of the FGF-8 gene was detected in testis but not in other adult tissues analysed. During development, expression of FGF-8 was restricted to embryonic days 9 through 13 suggesting that the growth factor plays a role during a discrete stage of mouse embryogenesis. An exon-containing genomic clone of human FGF-8 was isolated and structural comparisons indicated that the gene structure of this region is highly conserved among the FGF genes. Using a panel of human-rodent somatic cell hybrids, the FGF-8 gene was localized to human chromosome 10.