Immunoperoxidase stains demonstrate a consistent pattern of antigen expression in PPB but such staining, in general, adds little to the diagnostic assessment obtained by standard histologic studies. Muscle specific actin and desmin are consistently positive in the obvious rhabdomyoblasts and also decorate to a lesser degree the small primitive cells in the cambium layer zones. Blastematous islands occasionally show weak staining for muscle specific actin. Vimentin stains the rhabdomyoblastic cells, cartilage, fibrosarcoma, or fibrous histiocytoma-like foci, and focally decorates blastematous islands. Weak neuron-specific enolase staining of blastematous areas is rarely seen. S-100 protein staining is confined to areas of conspicuous cartilaginous differentiation, and cytokeratin staining is limited to the epithelium of the cystic spaces or entrapped respiratory or mesothelial elements.