Monoclonal antibodies derived from the actin multigene family are routinely used as an adjunct to morphologic diagnoses of smooth muscle tumors. Northern blot analysis was performed on 60 surgical resections utilizing isoactin-specific cDNAs. A comparison of this analysis to immunohistochemical studies demonstrated that actin-specific monoclonal antibodies represent reliable markers of the smooth muscle lineage. Smooth muscle neoplasms showed a unique pattern of gamma-smooth muscle isoactin gene expression, providing a potentially valuable molecular adjunct to the morphologic diagnosis of uterine smooth muscle tumors.