Calponin is a thin-filament-associated protein that has been implicated in the regulation of smooth-muscle contractility. It binds to F-actin and inhibits the MgATPase activity of actomyosin. In the present work we have examined the effect of recombinant chicken gizzard alpha-calponin (R alpha CaP) on the binding of rabbit skeletal-muscle myosin subfragment 1 (S1) to F-actin and on the inhibition of its actin-activated MgATPase. We have found that binding of one R alpha CaP molecule to every three to four actin monomers is sufficient for maximal inhibition of acto-S1 ATPase. At this R alpha CaP/actin ratio R alpha CaP does not interfere with S1 binding to F-actin. At higher concentrations, R alpha CaP displaces S1 from F-actin and a 1:1 R alpha CaP-actin monomer complex is formed. R alpha CaP is also able to displace troponin I from its complex with F-actin which may reflect the amino acid sequence similarity between R alpha CaP and troponin I in their actin-binding regions.