To better understand the regulation of striated muscle contraction, the effects of myosin subfragment 1 (S-1) on the actin binding of cardiac troponin and tropomyosin were investigated. Troponin's affinity for actin-tropomyosin was 4-fold stronger in the absence than in the presence of myosin S-1. CaCl2 had no effect on troponin binding to the thin filament in the presence of myosin S-1. The binding curve was weakly cooperative, implying interactions between adjacent troponin molecules. Myosin S-1 increased (40-200-fold) the affinity of tropomyosin for the thin filament, an effect opposite to the effect of myosin on troponin. This effect was highly cooperative and occurred in the presence of ADP or in the absence of nucleotide. Myosin altered the effect of ionic conditions on tropomyosin-actin binding, consistent with tropomyosin binding to a different site on F-actin in the presence of myosin. The results indicate that troponin-tropomyosin and strongly binding myosin cross-bridges do not compete for an F-actin binding site. Although repositioning of troponin-tropomyosin on the actin filament may be sterically required for tight myosin-actin binding, a myosin-induced conformational change in actin provides a better explanation for the complex effects of myosin on thin filament assembly.