PURPOSE: To obtain the subject promoting factor, useful for utilization to artificial production of honeybee queens and as medicines or functional foods and having a specific molecular weight by ultrafiltering a water-soluble protein prepared from honeybee royal jelly extracted with an ether.

CONSTITUTION: The objective promoting factor is obtained by extracting honeybee royal jelly with a hydrophobia organic solvent, ultrafiltering a water-soluble protein obtained from the residue through a membrane (e.g. YM-2 membrane (R)) and fractionating the molecular weight into 1000-10000. The resultant factor is then administered to larvae of honeybees to promote differentiation into queens.