We utilized a novel in vitro assay to investigate factors that modulate the behavioral phenotype of fibroblasts during wound healing. Using the Isometric Cell Traction Assay (ICTA), we quantified simultaneously the migration and traction of rat dermal fibroblasts entrapped in a collagen gel immersed in cell culture medium. We found that the addition of 10% day 1 wound fluid to culture medium inhibited fibroblast-mediated gel compaction when compared to medium with 10% rat serum or 10% day 5 rat wound fluid. Similar results were obtained after addition of 5-50 ng/ml platelet-derived growth factor-BB (PDGF-BB)+10% serum. These results suggest that PDGF-BB, a growth factor released by platelets and in abundance in early wound fluid, is an important species in stimulating phenotype transition to a migrating (early) wound fibroblast versus a tissue contracting (late) wound fibroblast.