Immunohistochemical study of neuroblastomas, Ewing sarcomas, rhabdomyosarcomas, and Wilms tumors demonstrate specific expression of peripherin and α-internexin in 20/22 and 6/22 cases of neuroblastomas, respectively. Microtubule-associated protein 1B (MAP 1B) was strongly and diffusely expressed in all 22 cases of neuroblastomas, but was also focally or multifocally expressed in 9/12 rhabdomyosarcomas and also in the blastema and stroma of 8/11 Wilms tumors. All rhabdomyosarcomas strongly and diffusely express nestin, but this marker was also expressed, multifocally, in 15/22 neuroblastomas and also in the blastema and stroma of all 11 Wilms tumors. NeuN, a neuron-specific nuclear protein, was expressed focally in 1 case of neuroblastoma and diffusely in 2 other cases (3/22). Surprisingly, it was also focally expressed in 2/12 rhabdomyosarcomas. In contrast, all 7 cases of Ewing sarcoma were negative for peripherin, MAP 1B, α-internexin, NeuN, and nestin. Thirteen neuroblastomas were also immunostained for neurofilaments, tyrosinase, and anaplastic lymphoma kinase 1 (ALK 1), and were found to be negative for these markers. Our results confirm that peripherin and α-internexin are neuroblastoma markers useful for the differential diagnostic work-up of small round cell tumors of childhood. Strong diffuse immunoreactivity for MAP 1B favors a diagnosis of neuroblastoma, whereas strong diffuse immunoreactivity for nestin favors a diagnosis of rhabdomyosarcoma.