The interaction of heavy-chain isoforms of myosin subfragment-1 with actin was examined by cross-linking with carbodiimide (EDC). The heavy chain of SI could be cross-linked to a single actin molecule through sites on either 20 or 50 kDa proteolytic domains, resulting in complexes which migrated in an 8% polyacrylamide gel in the presence of Tricine buffer with an apparent molecular mass (Mapp) of 150 or 160 kDa, respectively. Cross-linking of SI through both sites to two actins produced a complex migrating with an Mapp of 210 kDa. Cross-linking of the S1 (Al) isoform [but not S1(A2)] to F-actin produced four additional peptides with Mapp values of 64, 120, 185, and 235 kDa. These peptides corresponded to cross-linked complexes of A1+actin, S1HC+actin, A1+actin+S1HC, and A1+actin+S1HC+actin, respectively. The production of the 64, 160, 185, 210, and 235 kDa complexes was almost inhibited at a high degree of saturation while the inhibition of the 150 kDa product was relatively small. At a low degree of saturation, the ratio of 150 to 160 kDa complexes was 1. Crosslinking between the S1 isoforms and regulated F-actin was not affected by Ca2+. These data show that contact of the S1 to one actin protomer is through a site on the 20 kDa fragment and to the second actin protomer through the sites located on the 50 kDa fragment and on the essential light-chain 1. At nonphysiological conditions of full saturation of actin filaments with myosin heads, the binding of heavy chain at S1 and of A1 to the second actin could be almost abolished.