We have previously reported that tranilast, an anti-allergic agent, specifically suppresses collagen synthesis in normal skin fibroblasts and to a greater extent in keloid fibroblasts. We found in this study that the specific suppression of collagen synthesis by tranilast was limited to the fibroblasts with a high passage number (passage 8-10). In normal skin fibroblasts with a low passage number (passage 1-2), tranilast exerted no significant effect on collagen synthesis. This was also observed with scleroderma and keloid fibroblasts. This result suggests that inhibition of collagen by tranilast will be dependent on in vitro cellular aging and that serial cell passages result in the loss of the cell phenotype resistant to tranilast effect.