The nucleotide and deduced amino acid sequences of two isoforms of mouse smooth-muscle myosin heavy chain (SM1 and SM2) were determined. SM1 (6175 bp) and SM2 (6214 bp) cDNA contained a single open reading frame that encodes 1972 and 1938 amino acids (227,056 Da and 223,294 Da), respectively. Smooth muscle myosin heavy chain mRNA was expressed highly in smooth muscle tissue (small intestine) and weakly in heart and lung. Each of SM1 and SM2 cDNA was transfected and expressed in CHO cells. The expressed myosin heavy chains were detected with an antibody raised against smooth muscle myosin heavy chains and showed the same mobility as the native smooth muscle myosin heavy chains in SDS-PAGE.