The physiologic function of insulin in early embryonic life is unknown. We have shown that insulin is present in unfertilized eggs and in chick embryos at 2-3 days of development, even before the emergence of the endocrine pancreas. To define insulin's role, we exposed 2-day-old chick embryos to anti-insulin antibodies and followed their development up to day 5. Antibody-treated embryos had a higher rate of growth retardation and death by days 3-5 of embryogenesis, compared with controls. Among the survivors, biochemical maturation was delayed at days 4 and 5; weight, protein, total creatine kinase activity, and creatine kinase-MB were decreased in antibody-treated embryos. By contrast, insulin (50 ng/embryo) administered to 2-day-old embryos yielded nearly symmetrical stimulatory results. These findings suggest that endogenous insulin plays a probable physiologic role regulating growth and differentiation in early embryos. In addition, the findings provide some clues to a possible function for insulin produced outside the organism's own beta cells.