Samples of aqueous humor from rabbits and humans were examined for growth effects on sub-confluent monolayer fibroblast cultures from ocular and extraocular tissues. At a concentration of 20% aqueous by medium volume, both homologous and heterologous aqueous stimulated DNA synthesis in fibroblasts from both species. The stimulation was independent of medium serum. When fibroblasts were exposed to 0 to 100% aqueous in serum-free medium or 0 to 98% aqueous in 2% serum-supplemented medium, no concentration of aqueous was observed in which DNA synthesis was inhibited. However, higher concentrations of aqueous led to increasing cell loss resulting in an apparent net inhibition of culture growth. Net growth effects of aqueous humor were, therefore, dependent upon aqueous concentration and were apparently due to the combined effects of stimulation of proliferation and reduction in cell survival.