The contractile properties of muscle fibres are, in part, determined by the myosin heavy chain (MyHC) isoforms they express. Using monoclonal antibodies, we show that at least three forms of slow twitch MyHC accumulate sequentially during mouse fetal development and that slow MyHC maturation in slow fibres occurs before expression of the adult fast MyHCs in fast fibres. Expression of deletion derivatives of beta-cardiac MyHC cDNA shows that the slow MyHC epitopes that are detected in adult but not in young animals are located near the N-terminus. The same N-terminal region of various fast MyHC molecules contains a conserved epitope that can, on occasions, be observed when slow MyHC cDNA is expressed in non-muscle cells. The results raise the possibility that the N-terminal epitopes result from post-translational modification of the MyHC and that a sequence of slow and fast MyHC isoform post-translational modifications plays a significant role during development of murine muscle fibres.