In the honey bee, Apis mellifera, the fifth larval instar is a critical period for caste differentiation. During this premetamorphic phase the hormonal milieu shows pronounced caste differences and several organs, particularly the ovaries, enter different developmental pathways leading to highly fertile queens and nearly sterile workers. Developmental profiles of total protein synthesis in larval ovaries showed marked caste differences starting with the early fifth instar. By two-dimensional electrophoresis, caste-specific patterns could be detected in the synthesis of a 29 kDa/pI 4.6 and two 24 kDa/pI 5.2-5.5. proteins (pI=isoelectric point). A marked decrease in the expression of these proteins was found to coincide with caste-specific differences in the haemolymph ecdysteroid titer. In vitro exposure of larval worker ovaries to physiological (10 M) concentrations of synthetic makisterone A elicited an identical response. Juvenile hormone did not affect protein synthesis patterns in larval ovaries, and also did not inhibit or reverse the ecdysteroid-induced effects. Heat shock experiments revealed that the 29 kDa/pI 4.6 ecdysteroid-regulated protein belongs to the class of small heat shock proteins.