Human dermal fibroblasts were cultured with blister fluid and 3 of their properties were investigated: cell shape, growth rate, and ability to contract collagen fibrils. In the medium containing the blister fluid, fibroblasts became attached to the tissue culture plastic and became well elongated to a spindle form. The blister fluid demonstrated a potent fibroblast growth—promoting activity comparable to that observed in the serum obtained from the same patient. This activity was independent of the patient's age and the time elapsed since the burn. The growth rate of cells increased as the concentration rose to 20% in the case of serum. Growth promotion by the blister fluid, however, was maximum at the concentration of 2.5%, leveling off thereafter, a fact which suggests the presence of some other factor possessing an inhibitory action on fibroblast growth. The growth-promoting activity of the blister fluid was not destroyed by tryptic digestion. The blister fluid promoted a fibroblast-mediated contraction of collagen fibrils to an extent comparable to that of the serum. The protein-banding pattern of the fluid, obtained by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate, was similar to that of the serum. These results suggest that the fluid is an exudate mainly from the vascular system, that it provides a good environment for the fibroblasts in the damaged site, and that it facilitates the healing process.