In the stage 4 chick blastoderm, an area located 0.6 mm posterior to Hensen's node, the post-nodal piece (PNP), consists of an undifferentiated population of cells, since the explants when cultivated in vitro in a variety of media do not develop into any histologically identifiable structures. However, addition of a specific low molecular weight RNA isolated from the 16-day-old chick embryonic heart promotes the appearance of a distinct mode of morphological and biochemical changes that is similar to that of embryonic cardiogenic process. The RNA-induced changes in the PNP also include a marked increase in acetylcholinesterase activity. The increase in enzymatic activity can be measured biochemically, as well as visualized histochemically.