Antibodies were produced against myosins isolated from the left atrial myocardium (anti-bAm) and the left ventricular myocardium (anti-bVm) of the bovine heart. Cross-reactive antibodies were removed by cross-absorption. Absorbed anti-bAm and anti-bVm were specific for the myosin heavy chains when tested by enzyme immunoassay combined with SDS gel electrophoresis. Indirect immunofluorescence was used to determine the reactivity of atrial muscle fibers to the two antibodies. Three populations of atrial muscle fibers were distinguished in the bovine heart: (a) fibers reactive with anti-bAm and unreactive with anti-bVm, like most fibers in the left atrium; (b) fibers reactive with both antibodies, especially numerous in the right atrium; (c) fibers reactive with anti-bVm and unreactive with anti-bAm, present only in the interatrial septum and in specific regions of the right atrium, such as the crista terminalis. These findings can be accounted for by postulating the existence of two distinct types of atrial myosin heavy chains, one of which is antigenically related to ventricular myosin. The tendency for fibers labeled by anti-bVm to occur frequently in bundles and their preferential distribution in the crista terminalis, namely along one of the main conduction pathways between the sinus node and the atrioventricular node, and in the interatrial septum, where different internodal tracts are known to converge, suggests that these fibers may be specialized for faster conduction.

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