Rhabdomyosarcomas were observed at birth in two premature infants. The evolution of these tumors was both rapid and fatal. One case was a botryoid sarcoma of the bladder and the second was a cervical alveolar sarcoma of Riopelle and Thériault. A cytological differentiation of the tumor was observed in the first case spontaneously and in the second following chemotherapy. In the second case histology, immunocytochemistry and a novel biochemical analysis of the contractile proteins demonstrated this differentiation of the tumor. The cellular reactivity varies according to the degree of muscular differentiation that can be defined either by studying the type of intermediate filaments present or the expression of specific muscle contractile proteins.