The in vitro life span and rate of growth of calf lens cells cultured in serum-supplemented 199 medium can be markedly increased by growing the cells with a layer of mitomycin -killed 3T6 feeders. In the absence of feeders, the epithelial cells are partially blocked in the S period of the cell cycle but show a normal distribution of cells in G1 and G2 when grown with fibroblasts. Increased growth rates and division potential can also be achieved by growing the lens in 199 medium containing 10(-5) M thymidine, and cells grown under these conditions show a normal growth cycle. Our results suggest that lens epithelial cells cultured in medium 199 show a deficient endogenous synthesis of thymidylic acid, and fibroblast feeders or exogenously added thymidine enable them to overcome this deficiency. When grown in the presenct of 10(-5) M thymidine, the lens epithelial cells show a very low serum requirement for cell division in short term culture.