The effect of ascorbate and L-ascorbic acid 2-phosphate (P-Asc), a stable derivative of ascorbate, on the growth of cultured rabbit keratocytes was examined. In a 12-day culture, P-Asc (0.05-0.1 mM) and ascorbate (0.05 mM) enhanced the growth of the cells, whereas 0.1-1.0 mM ascorbate was cytotoxic to the cells. An increase in the concentration of P-Asc up to 1.0 mM resulted in no further change in the cell growth. The effect of 0.1 mM P-Asc was observed after the confluency had been achieved in the culture. The lower toxicity of P-Asc may be due to its stability. The presence of both 0.1 mM P-Asc and mouse epidermal growth factor in the medium stimulated cell growth to a higher degree than each factor alone. The results suggested that these two factors enhance the growth of keratocytes in a different manner.