Growth factor-stimulated cell proliferation is preceded by a transient increase in the cytoplasmic Ca2+ concentration, and calmodulin, an intracellular calcium receptor protein, appears to intervene in the modulation of this process. Calmodulin binds to nuclear proteins and could therefore affect their functions However, calmodulin does not control cell proliferation by acting exclusively at the nuclear level. Several lines of evidence obtained in our laboratory indicate that the epidermal growth factor receptor (EGFR) is a calmodulin-binding protein and that calmodulin acts as a modulator of its intrinsic tyrosine kinase activity.Instituto de Investigaciones Biomédicas Consejo Superior de Investigaciones Científicas Arturo Duperier 4 28029 Madrid, Spain. Centro de Biología Celular Facultad de Ciencias Universidad Central de Venezuela Caracas, Venezuela