1. An effective technique is described for the aseptic culture of larvae of Drosophila melanogaster.
2. Factors which accelerate the growth of Drosophila in aseptic culture on a basal medium containing casein as a source of amino acids can be extracted from both yeast and pancreas in water-soluble form.
3. Part of the activity of such fractions can be removed by adsorption on anion exchange resins, or on charcoal; and eluted with ammonia.
4. The whole activity is dialysable and recoverable from the dialysate by evaporation under reduced pressure.
5. The active components are heat stable.
6. The active fraction is insoluble in N-butanol, ethanol, acetone, pyridine and ether.
7. Some evidence is presented for the presence of at least two components.
8. The possibility that one of the factors may be a peptide is considered. It is thought unlikely that it is an amino acid.