The influence of platelet-rich plasma lysates on fibroblast proliferation was studied in culture. Cells were exposed to platelet lysates that had been preincubated at different pHs (5.0, 7.1, and 7.6). Proliferation was evaluated with the MTT assay and incorporation of [3H]thymidine into macromolecules, while type I collagen production was assayed by Western blotting. Enzyme-linked immunosorbent assays were used to determine platelet-derived growth factor and transforming growth factor-β concentrations. Platelets preincubated in an acidic environment (pH 5.0) induced the highest degree of fibroblast proliferation, and the concentration of platelet-derived growth factor in the different treated lysates was the highest at that particular pH. The concentration of transforming growth factor-β, however, was lower after incubation at pH 5.0 than at either pH 7.1 or 7.6. These findings may be relevant to normal wound healing in vivo and useful in the treatment of wounds and delayed healing processes. (WOUND REP REG 2002;10:336–340)