The effect of caltropin (CaT) on the caldesmon (CaD)-G-actin interaction was monitored by viscosity measurements, bioassays measuring the release of inorganic phosphate (Pi) following G-actin polymerization and fluorescence studies using acrylodan labelled G-actin. CaD induced polymerization of G-actin into filaments in the absence of salt was accompanied by an increase in relative viscosity. This effect of CaD was essentially abolished by CaT in the presence of Ca2+. In bioassays the rate of Pi release was reduced significantly in the presence of Ca2+/CaT. Acrylodan labelled G-actin when excited at 375 nm exhibited an emission maximum at 478 nm. Polymerization of G-actin resulted in shifting the emission maximum to 465 nm. When CaD was added to G-actin containing Ca2+/CaT, the rate of G-actin polymerization was reduced considerably, suggesting that CaT interferes in the CaD-G-actin interaction.