Rabbit and frog lens epithelial cells can be induced to undergo DNA synthesis and mitosis in a completely defined insulin containing medium. The hormone was added to Medium 199 in these experiments. In general, the insulin aroused division throughout the central region of the epithelium. Insulin added to Medium 199 from a different source with a different formulation, was unable to evoke the response. The non-stimulating 199, could support the activation if it was supplemented with insulin plus rabbit serum dialysate or the low molecular weight serum components obtained by gel filtration. In contrast to the response elicited by whole rabbit serum, marked cellular disorganization did not precede the hyperplasia that occured after culture in Medium 199 plus insulin. The temporal aspects of DNA synthesis and mitosis observed in the presence of insulin, are similar to those obtained employing whole rabbit serum. These experiments have led to the fabrication of a system (for mammalian and amphibian material) in which an organized tissue can be prompted to increase, strikingly, its level of mitosis, while being cultured in a medium whose constituents are known.