Normal development of the chicken embryo requires insulin and insulin receptors. Insulin and also insulinlike growth factor I (IGF-I) can stimulate embryonic growth when applied in vivoat the beginning of organogenesis (Girbau, M., Gomez, J. A., Lesniak, M. A., and De Pablo, F. (1987) Endocrinology121, 1477–1482). In the present work we chose the developing eye lens, an avascular organ composed of a single cell type, to characterize further the specific effects of insulin and IGF-I upon cell differentiation and gene expression. Epithelial cells (before terminal differentiation) and fiber cells (terminally differentiated) were cultured in the presence of the hormones. δ-Crystallin mRNA steady-state levels as well as nuclear d-erystallin gene transcription were measured. Either insulin or IGF-I (0.1–10 ng/ml) increased (2–4-fold) 5-crystallin mRNA in epithelial and fiber lens cells from day 6 embryos. The effect of insulin was largely blocked by the Fab fragment of anti-insulin receptor antibody (B-10). By contrast, as it had been shown for metabolic actions in other systems, bivalent B-10 IgG itself mimicked insulin action, i.e. it induced an increase on δ-crystallin mRNA levels. Thus, insulin appears to act through its own receptor in regulating the levels of δ-crystallin mRNA. There was a differential transcriptional component in insulin and IGF-I effects on δ-crystallin gene expression, IGF-I induction of transcription, as measured by nuclear run-on assay, is greater than insulin induction (≈2.5-foid versus1.4-fold) and faster. The δ-crystallin gene will provide the opportunity to analyze the action of insulin and IGF-I on the expression of a structural protein marker of cell differentiation during early embryonic development.