Two cases of 'adult' rhabdomyoma and 2 of 'fetal' rhabdomyoma have been immunologically studied using several antisera specific of skeletal muscle fibers proteins. This paper also deals with a review of the literature on these rare benign skeletal muscle proliferations. It is concluded that 'adult' rhabdomyoma contains fetal myosin and therefore the term 'adult' does not seem immunologically appropriate any longer. In addition the tumoral cells show the same level of differentiation as seen in neonatal skeletal muscle. Therefore it seems that the definition of neonatal rhabdomyoma is more appropriate for these benign muscular proliferations. The 2 cases of fetal rhabdomyoma appear phenotypically similar to normally developing myoblasts and contain fetal myosin. Therefore it seems that the definition 'fetal' is here appropriate for what appears to be a disorganized proliferation of otherwise normal fetal skeletal muscle fibers. The differential diagnosis between these latter tumors and rhabdomyosarcomas has been discussed.