Basic and acidic fibroblast growth factor (FGF) are related both structurally and functionally. A bovine basic FGF cDNA and a human acidic FGF genomic fragment were used as hybridization probes in Southern blot analysis of DNAs isolated from a panel of 30 mouse-human cell hybrids. The gene encoding basic FGF was assigned to human chromosome 4, and the gene for acidic FGF to human chromosome 5. The two growth factors which are presumed to have a common evolutionary ancestor are therefore not linked. A HindIII restriction fragment length polymorphism was detected for human basic FGF.