The binding of heavy meromyosin (HMM) to F-actin was examined at varying ionic strengths and temperatures and in the presence of ADP and AMPPNP and then compared to the binding of subfragment 1 (S-1) under identical conditions. In the absence of nucleotide (mu = 0.12-0.43 M, 22 degrees C), HMM binds 100-1000-fold more strongly to F-actin than does S-1. This indicates that, in the absence of nucleotide, both heads of HMM bind to F-actin, with the second head making a significant contribution to the free energy of binding. On the other hand, in the presence of ADP (mu = 0.43 M, 22 degrees C) or AMPPNP (mu = 0.12 M, degrees C), the binding of HMM to F-actin is quite similar to the binding of S-1, indicating that here the second head of HMM does not make a strong contribution to the free energy of binding. In fact, in the presence of AMPPNP, HMM appears to bind to F-actin primarily with one head, while the detached head may be interfering with the binding of another HMM molecule at an adjacent actin site. With all of the different agents tested (ionic strength, temperature, and nucleotide), the effect of the agent on the binding of HMM to F-actin is approximately the square of its effect on the binding of S-1 to F-actin, results consistent with these various agents affecting the binding of each of the two HMM heads to the same extent as they affect the binding of an S-1 head.