Loss of maturation features is demonstrated for 8-day-old chick embryo heart myocytes, once they have been completely dissociated by trypsin. In support of this statement a total of 65 sections of six isolated cells, fixed while still spherical or during early flattening, were examined under the electron microscope. Trypsin-separated heart muscle cells, even though originating from already differentiated embryonic heart tissue, can therefore in principle be used for differentiation experiments in culture. However, the same cell suspensions also yielded an appreciable quantity of nonisolated cells. In such cell complexes, one can find areas showing well-ordered fibrils and intercalated disks. From 27 sections of a cell pair incidentally transferred into culture undissociated and then fixed while still in the globular state, the fourth and fifth sections, starting from the substrate side of the culture, showed an intercalated disk. Because of its small diameter, this cell complex would hardly have been retainable by a gauze with meshes likely to allow passage of only single cells. Thus the availability of differentiation experiments in culture, starting with already differentiated heart tissue, is restricted to cases where, in a selected territory, each cell has been established without doubt as isolated.