Acidic fibroblast growth factor (aFGF) binds to specific transmembrane receptors and is partly transported to a nuclear location. To study this transport we made a kinase-negative mutant of FGF receptor 4 as well as one where the major part of the cytoplasmic receptor domain was deleted, and expressed them in U2OSDr1 cells that lack functional FGF receptors. All receptors mediated endocytic uptake of aFGF. Translocation of the growth factor across cellular membranes was assayed using aFGF with a C-terminal CAAX-motif, which signals addition of a farnesyl group onto the protein once in the cytosol. CAAX-tagged aFGF was farnesylated when incubated with cells containing wild-type or kinase-negative receptors. It was not farnesylated in cells expressing the deleted receptor, or when the incubation was in the presence of genistein. aFGF incubated with cells transfected with wild-type or kinase-negative receptors, but not with the deleted receptor, was partly recovered from the nuclear fraction in the absence, but not in the presence of genistein. The data indicate that the cytoplasmic receptor domain, but not the active kinase, is required for transport of the growth factor into cells, and that genistein inhibits the process.