Stem cells indigenous to the cancellous spaces of the bone bed in an acute injury provide an important source of pluripotent cells for cartilage repair. Insulin-like growth factor-I facilitates chondrogenesis of bone marrow-derived stem cells in long-term culture and may enhance chondrogenesis in healing cartilage lesions in vivo. This study examined the impact of insulin-like growth factor-I, gradually released from fibrin clots polymerized in situ, on the recruitable stem-cell pool in a full-thickness critical cartilage defect model. Twelve full-thickness 15-mm cartilage lesions in the femoropatellar articulations of six young mature horses were repaired by an injection of autogenous fibrin containing 25 microg of human recombinant insulin-like growth factor-I or, in control joints, fibrin without the growth factor. All horses were killed at 6 months, and cartilage repair tissue and surrounding cartilage were assessed by histology, histochemistry, types I and II collagen immunohistochemistry, types I and II collagen in situ hybridization, and matrix biochemical determinations. White tissue filled grafted and control lesions, with the growth factor-treated defects being more completely filled and securely attached to the subchondral bone. A moderately improved chondrocyte population, more columnar cellular organization, and better attachment to the underlying bone were evident on histological evaluation of growth factor-treated defects. Type-II procollagen mRNA was abundantly present in the deeper half of the treated sections compared with moderate message expression in control tissues. Immunolocalization of type-II collagen showed a preponderance of the collagen in growth factor-treated lesions, confirming translation of type-II message to protein. Composite histologic healing scores for treated defects were significantly improved over those for control defects. DNA content in the cartilage defects was similar in treated and control joints. Matrix proteoglycan content was similar in treated and control defects and lower in the defects than in the intact surrounding and remote cartilage of the treated and control joints. The proportion of type-II collagen significantly increased in growth factor-treated tissues. Fibrin polymers laden with insulin-like growth factor-I improved the histologic appearance and the proportion of type-II collagen in healing, full-thickness cartilage lesions. However, none of the biochemical or morphologic features were consistent with those of normal articular cartilage.