We have used the sulfhydryl-specific heterobifunctional photo-cross-linker 4-maleimidobenzophenone (BP-Mal) to study the interactions of rabbit skeletal tropomyosin with troponin and of the troponin subunits with each other. We found that alpha,alpha-tropomyosin specifically labeled at Cys-190 with BP-Mal photo-cross-links with all three subunits of troponin with decreasing cross-linking yields in the order of troponin T, troponin I, and troponin C. There was no apparent Ca2+ dependence in the cross-linking yields. In separate experiments, we found that troponin C labeled specifically at Cys-98 with BP-Mal photo-cross-links to both troponin I and troponin T in the two binary complexes, as well as in the ternary complex. Again, no Ca2+-dependent changes in the cross-linking yields were detectable. These results are in general agreement with the picture that troponin I and troponin T are in close contact with troponin C near its Cys-98 and that all three troponin subunits are in the proximity of Cys-190 of tropomyosin.