The extent to which the association between insulin-like growth factors (IGFs) and their specific binding proteins (BPs) prevents their crossing the capillary barrier was studied by comparing their distribution in serum with that in samples of lymph collected from the lower leg of five subjects undergoing radiographical investigation of the lymphatic system. The IGF concentrations in lymph were 10-30% of the corresponding serum levels, and in each subject the ratios of IGF-I and IGF-II in the lymph to those in the serum were similar. Western blot analysis of the BPs revealed that the five molecular forms identified in serum also were present in lymph, but in significantly smaller quantities. The 41.5K and 38.5K forms, which constitute the binding units of the large complex (approximately 150K) of serum and are also capable of binding IGFs in monomeric form, were present in smaller amounts than the 34K, 30K and 24K forms, which belong specifically to the small complex (approximately 40K) of serum. The BPs extracted from lymph were similar to those of the small complex, with a preferential affinity for IGF-II and only half of the affinity for IGF-I of the BPs extracted from serum. With neutral pH gel filtration of lymph, more than 90% of IGFs and binding activity eluted with the material in the area of the 40K zone. These data indicate that the 150K IGF-BP complexes do not cross the capillary barrier, whereas the 40K complexes do. The function of the former may be to provide a reservoir and buffering action of the IGFs, whereas the latter may be involved in the transport of the IGFs to their target cells.