A specific inhibitory factor was isolated from human serum of a patient who manifested impaired prosthetic graft tissue incorporation. The later reaction was hypothesized to be related to inhibiting fibroblast proliferation by a specific humoral factor. The crude inhibitory serum was tested against a pool of normal human sera with different cell types in culture. Proliferation of human skin fibroblast and human smooth muscle cell cultures incubated in the presence of 50% inhibitory serum were inhibited up to 58% and 37%, respectively. Proliferation of bovine capillary endothelial cell cultures was stimulated under similar conditions. Isolation and purification of the inhibitory factor from crude serum were initiated by ammonium sulfate precipitation. The pellet was further fractionated by sepharose 6B gel filtration. The inhibitory activity was eluted from the column in a relatively purified form as indicated by gel electrophoresis of the inhibitory fractions, which demonstrated a specific band corresponding to the inhibitory protein, with an apparent relative molecular mass of 230 kDa. The inhibitory factor showed a high affinity to concanavalin A, indicating its nature as a glycoprotein not associated with albumin or immunoglobulin fractions of the serum.