Despite extensive investigation of the insulin-like growth factor (IGF)/IGF-binding protein (IGFBP) system in the circulation and body fluids, there is no information on this in interstitial fluid. We have compared the IGF/IGFBP system in the circulation with that in fluid obtained from blisters artificially raised by negative pressure in 10 healthy volunteers. IGFBP-1, -2, -3, and -4 were all found in blister fluid, but in concentrations much lower than those in matched serum. The IGF-I, IGF-II, and IGFBP-3 levels measured by RIA were 18%, 14%, and 16% of those in serum, respectively. Fast protein liquid chromoatography showed that both IGF-I and IGFBP-3 in 150- and 50-kilodalton complexes were approximately 13% and 37%, respectively, of the corresponding peaks found in matched serum. Compared to that in serum, the IGFBP-3 in the blister fluid was predominantly in a modified 29-kilodalton form, and there was increased activity of an IGFBP-3 protease. Therefore, although IGF concentrations are much lower in interstitial fluid than in the circulation, a greater proportion of this IGF is in forms more readily available for interaction with tissue receptors. The blister fluid appears to represent physiological interstitial fluid and may provide a model for studying the physiology and pathophysiology of growth factors in the interstitial environment.