The identification of acidic and basic fibroblast growth factors (FGFs) in a number of embryonic tissue extracts has implicated these growth factors in the regulation of a variety of embryonic events including angiogenesis, eye development, and muscle differentiation. Lack of information concerning the cellular distribution of the growth factor within these tissues has made it extremely difficult to assign developmental roles to FGF. We have localized bFGF in the developing chick embryo using immunohistochemical techniques and our monospecific polyclonal rabbit anti-human bFGF IgG. The spatial pattern for bFGF localization was highly specific. The anti-human bFGF antibodies recognized striated muscle cells and their precursors in 2-6-d chick embryos. Myocardium, somite myotome, and limb bud muscle all stain positively for bFGF. In addition, the anti-human bFGF antibodies localized specifically to the cell, rather than to the extracellular matrix or nucleus of myotubes. The localization of bFGF demonstrated here provides further support for the hypothesis (Clegg et al., 1987; Seed et al., 1988) that this growth factor is involved in muscle development.