Ascorbic acid has been reported to promote the differentiation of embryonic stem (ES) cells into cardiomyocytes; however, the specific functions of ascorbic acid have not been defined. A stable form of ascorbic acid, namely, L-ascorbic acid 2-phosphate (A2-P), significantly enhanced cardiac differentiation; this was assessed by spontaneous beating of cardiomyocytes and expression of cardiac-specific markers obtained from mouse ES cells. This effect of ascorbic acid was observed only when A2-P was present during the early phase of differentiation. Treatment with two types of collagen synthesis inhibitors, L-2-azetidine carboxylic acid and cis-4-hydroxy-D-proline, significantly inhibited the A2-P-enhanced cardiac differentiation, whereas treatment with the antioxidant N-acetyl cysteine showed no effect. These findings demonstrated that ascorbic acid enhances differentiation of ES cells into cardiomyocytes through collagen synthesis and suggest its potential in the modification of cardiac differentiation of ES cells. (c) 2006 Elsevier Inc. All rights reserved.