The concentration of free insulin-like growth Factor-I (IGF-I) and its complexes was determined in human normal and osteoarthritic synovial fluids, using ultrafiltration through 20- and 100-kDa membranes, followed by a radioimmunoassay of each fraction. In addition, freshly obtained samples of normal and osteoarthritic cartilage were incubated for several days, at both 4 and 37 degrees C. The incubation media (desorbates) were analyzed the same way as the synovial fluid samples to yield the concentration of IGF-I in cartilage in situ. Our findings are (i) Free IGF-I content is extremely low in both human serum and synovial fluid and there is no significant difference between the two; (ii) The concentration of total IGF-I in normal human synovial fluid is an order of magnitude lower than that in serum due mainly to the decrease in the concentration of the large complex; (iii) Preliminary results show that the total IGF-I in osteoarthritic synovial fluids is twice as high as in normal fluids; (iv) In normal human cartilage the levels of IGF-I in all its forms are very low and are consistent with the expected exclusion of large molecules by the extracellular matrix; (v) By contrast, in osteoarthritic cartilage, the concentrations of all forms of IGF-I are high, probably due to increased permeability of the matrix and binding; (vi) The levels of IGF-I found in normal human cartilage are more than an order of magnitude lower than those which stimulate proteoglycan synthesis in human cartilage in culture, while the IGF-I levels in osteoarthritic cartilage lie in the range in which stimulation does occur.