Eleven human primary rhabdomyosarcomas (RMSs), including all histological variants, were analyzed morphologically, immunohistochemically for intermediate filament proteins and actin isoforms, and by means of Northern blots with probes specific for total actin, alpha-skeletal (SK), alpha-cardiac (CARD), and alpha-smooth muscle actin messenger (m)RNAs. All tumors disclosed ultrastructural evidence of skeletal muscle features with terminal differentiation in three cases. The RMSs contained immunohistochemically the intermediate filament proteins vimentin and desmin and reacted positively with the alpha-sarcomeric actin antibody, which recognizes alpha-SK and alpha-CARD actin isoforms. All RMSs reacted with the total actin probe, recognizing at 2.1 kb cytoplasmic actin mRNAs and at 1.7 kb alpha-actin mRNAs. With the specific probes, all RMSs expressed alpha-CARD actin mRNA, four neoplasms expressed also alpha-smooth muscle actin mRNA, whereas the probe for alpha-SK actin mRNA never produced a signal except in one case, in which the tumor masses were intermingled with non-neoplastic preexistent striated muscle fibers. Because alpha-CARD and alpha-smooth muscle actins are transiently expressed during normal skeletal muscle development, RMSs seem to follow normal skeletal myogenesis without completing the final step, consisting of alpha-SK actin mRNA expression. The use of Northern blots for alpha-CARD actin as an adjunct to conventional techniques may be helpful for the precise identification of primary RMSs compared to other soft tissue neoplasms.