Actin-binding proteins such as profilins participate in the restructuration of the actin cytoskeleton in plant cells. Profilins are ubiquitous actin-, polyproline-, and inositol phospholipid-binding proteins, which in plants are encoded by multigene families. By 2D-PAGE and immunoblotting, we detected as much as five profilin isoforms in crude extracts from nodules of Phaseolus vulgaris. However, by immunoprecipitation and gel electrophoresis of in vitro translation products from nodule RNA, only the most basic isoform of those found in nodule extracts, was detected. Furthermore, a bean profilin cDNA probe hybridised to genomic DNA digested with different restriction enzymes, showed either a single or two bands. These data indicate that profilin in P. vulgaris is encoded by only two genes. In root nodules only one gene is expressed, and a single profilin transcript gives rise to multiple profilin isoforms by post-translational modifications of the protein. By in vivo 32P-labelling and immunoprecipitation with both, antiprofilin and antiphosphotyrosine-specific antibodies, we found that profilin is phosphorylated on tyrosine residues. Since chemical (TLC) and immunological analyses, as well as plant tyrosine phosphatase (AtPTP1) treatments of profilin indicated that tyrosine residues were phosphorylated, we concluded that tyrosine kinases must exist in plants. This finding will focus research on tyrosine kinases/tyrosine phosphatases that could participate in novel regulatory functions/pathways, involving not only this multifunctional cytoskeletal protein, but other plant proteins.