We have established that the monoclonal antibody (MAb) AA21, raised against a crude sarcolemmal fraction prepared from adult chicken anterior latissimus dorsi muscle, recognizes the slow twitch/cardiac isoform of calcium ATPase. This was done using a combination of immunohistochemistry at the light and electron microscopic level, the change in the cell distribution in skeletal muscle during development, the molecular weight of the principal protein recognized in Western transfers, and direct comparison with another MAb of known specificity. The antigen is initially expressed by all myotubes at E10 and with development is gradually lost from all presumptive fast fibers. In addition to its immunoreaction and slow extrafusal skeletal muscle fibers, AA21 displays a highly selective immunoreactivity with a number of other cell types in different tissues. The antibody stains a subset of intrafusal muscle fibers and intestinal and arterial smooth muscle, but not venous smooth muscle. In the nervous system, a subpopulation of neurons is intensely stained, most neurons are faintly stained, and glia are not stained at all.