We investigated the Thy-1-like immunoreactivity during the development of chicken skeletal muscle using a group of monoclonal antibodies raised and characterised against purified chicken brain Thy-1. The immunoreactivity attributable to bona fide Thy-1 in muscle was present in nerves, connective tissue associated with the intrafusal capsule and blood vessels, and the extracellular matrix of the muscle fibres. During development there was no change in the staining of nerves, blood vessels or intrafusal capsules. However, the extracellular staining of muscle first appeared around hatching and gradually increased in intensity reaching maximal levels in the adult. The intensity of staining varied within and between the muscles examined. One of the antibodies (SB1 20.11) recognised an additional molecule that is not Thy-1 and that was localised in the cytoplasm of slow muscle fibres. This immunoreactivity was first detectable at E10 in all myotubes that contained both alkali and acid stable myosin ATPase activity (presumptive slow), but not in those myotubes with only alkali-stable myosin ATPase activity (presumptive fast). Thereafter, the staining increased to a maximum in the newly hatched animal and then decreased until reactivity was undetectable in the adult (greater than 25 weeks). All positive fibres initially stained with a uniform intensity but the time of commencement and the rate of loss of staining was variable. Those fibres that contained both acid stable and acid labile myosin ATPase activity lost the antigen much faster than the fibres containing only acid-stable myosin ATPase activity, which also tended to increase in intensity for a longer period. These may represent, respectively, the slow tonic type III fibres and the slow twitch type I fibres classified by Barnard et al. (1982).