Two skeletal myosin monoclonal antibodies, raised against human skeletal myosin, were used to study the correlation between function, primary and tertiary structure of S-1 prepared from rabbit skeletal myosin. The heavy chain of S-1 is cleaved into three fragments by trypsin--27 kDa, 50 kDa and 20 kDa--aligned in this order from the N-terminus. The epitope of the first antibody was assigned to the N-terminal 1-23 amino acid stretch of S-1, since it reacted with the 27 kDa N-terminal tryptic fragment of S-1 but not with a derivative of the 27 kDa fragment, which lacks the above amino acid stretch. The epitope of the second antibody was assigned to the 3 kDa N-terminal region of the central 50 kDa domain of S-1. This assignment was based on proteolytic and photochemical cleavage of S-1 and on the labelling of its N-terminus by a specific antibody. The antibodies were visualized binding to the myosin head on electron micrographs of rotary-shadowed complexes of antibodies with myosin. Measurements on the micrographs indicated that the distances between the head-tail junction of myosin and the 'anti-27 K' and 'anti-50 K' epitopes are 14 nm and 17 nm, respectively. Both antibodies have a high affinity to S-1. The affinity of the 'anti-50 K' to S-1 decreased upon actin binding, while that of the 'anti-27 K' was not affected by binding of S-1 to F-actin. The 'anti-50 K' antibody inhibited the K+ (EDTA) and the actin-activated ATPase activity of S-1, while the 'anti-27 K' had no effect. The results indicate that either the epitope of the 'anti-50 K' is near to the actin or to the ATP-binding sites of S-1, or that there is communication, expressed as propagated conformational changes, between these sites and the epitope.