Yeast extract, a component of Drosophila cell culture media, is shown to contain substances of high, intermediate, and low molecular weight, that are, respectively, essential, inhibitory, and stimulatory for colony formation in semisolid agar medium. Furthermore, it is shown that high concentrations of pyridoxal greatly increase the cloning efficiency of Drosophilia cells. A cloning method with line Kc is described which routinely gives cloning efficiencies in excess of 20%.