The apical ectodermal ridge permits growth and elongation of amniote limb buds; removal causes rapid changes in mesodermal gene expression, patterned cell death, and truncation of the limb. Ectopic fibroblast growth factor (FGF)-2 supplied to the chick apical bud mesoderm after ridge removal will sustain normal gene expression and cell viability, and allow relatively normal limb development. A bioassay for FGFs demonstrated that FGF-2 was the only detectable FGF in chick limb bud extracts. By distribution and bioactivity, FGF-2 is the prime candidate for the chick limb bud apical ridge growth signal.