In the present work we report on the role of a polyomavirus (Py) early function in interfering with both morphological and biochemical differentiation of the myogenic C2 cell line. The analysis of cell clones stably transfected with a plasmid carrying an ORI- Py genome showed that in the presence of the whole viral early region myogenesis is blocked and a transformed phenotype is evident. By using a plasmid that only encodes large-T function, the involvement of this individual early viral gene product was determined. Inhibition of myogenic differentiation by Py large T is proportional to the level of its expression. This inhibition does not appear to require alteration of cell growth properties. The analysis of muscle-specific functions expressed at different steps in the myogenic pathway showed that Py large T blocks the expression of terminal differentiation markers without altering the expression of the regulatory gene MyoD.