Cultures of Drosophila embryonic cells offer new opportunities for studying myoneural junctions. In culture, neuroblasts and myoblasts differentiate and yield neurons and myocytes. Some axons grow across the surface of the culture vessel and attach to myocytes, forming functional myoneural junctions. Therefore, all stages in junction formation may be examined in vitro under conditions where pharmacological, electrophysiological, and other commonplace approaches are facilitated. This method offers an additional, most powerful approach for studying the junctions, that of genetic analysis. Drosophila mutations may be sought which affect junction formation and function. Altered cells and junctions from mutants may then be compared to those from wild-type animals in order to dissect the gene-directed steps underlying junction phenomena.