The phosphorylation-dephosphorylation of serine and threonine residues of calponin is known tomodulatein vitro its interactionwithF-actin and is thought to regulate several biological processes in cells, involving either of the calponin isoforms. Here, we identify, for the first time, tyrosine-phosphorylated calponin h3 within COS 7 cells, before and after their transfection with the pSV vector containing cDNA encoding the cytoplasmic, Src-related, tyrosine kin-ase, Fyn.