The binding of troponin-I to tropomyosin, as well as its effects on the binding of troponin-T and its fragments T1 (residues 1-158) and T2 (residues 159-259) to tropomyosin, have been studied using immobilized alpha-tropomyosin. When applied alone, troponin-I exhibited weak interaction with tropomyosin and was eluted with a NaCl gradient at 0.12 M. Intact troponin-T was eluted at 0.40 M NaCl, while its fragment T1 was eluted from site 1 (near the COOH terminus of tropomyosin) at 0.32 M, independently from T2, which was eluted from site 2 (near Cys-190) at 0.22 M NaCl. However, the simultaneous presence of troponin-I and T2 resulted in formation of a strong troponin-I/T2/tropomyosin ternary complex at site 2 such that troponin-I/T2 complex was now eluted at 0.45 to 0.5 M NaCl. This binding was Ca2+-sensitive in the presence of troponin-C. An additional effect of troponin-I binding at site 2 was the strengthening of the T1/tropomyosin interaction at site 1, such that T1 was now eluted at the higher value of 0.45 to 0.50 M NaCl. Troponin-I also enhanced the binding of intact troponin-T to tropomyosin. These results suggest that cooperativity exists between the two sites, presumably induced by the binding of troponin-I to tropomyosin and mediated by a conformational change in the latter.