Pig heart myosins isolated from the free wall of the right ventricle and the free wall of the left ventricle were compared with respect to structural and enzymatic properties. The following parameters were studied (1) activation of myosin ATase by Ca2+ and K+j(2) molecular weight of the heavy and light chains of myosins as determined by electrophoretic migration in polyacrylamide sodium dodecyl sulfate (SDS) gels; (3) ability of the heavy chains to form aggregates at low ionic strength as revealed by electron microscopy; (4) sensitivity to the action of chymotrypsin. Differences were observed between left and right ventricular myosins (L-myosin and R-myosin) for all these parameters except for the molecular weight of heavy and light chains. The existence of large amounts of short synthetic filaments for R-myosin compared with L-myosin as revealed by the length repartition of the filaments, and the production of smaller quantities of HMM-S by chymotryptic digestion for R-myosin, strongly suggest the presence of different cardiac myosin heavy chain species.