To improve the understanding of factors with the potential of affecting the healing of flexor tendons, this study compared the cellular effects of recombinant human insulin-like growth factor-II with those of recombinant human insulin-like growth factor-I in matched pairs of deep flexor tendons of young rabbits. Dose-response effects on the synthesis of DNA and matrix proteins of either factor alone or in combination were investigated in short-term culture, and effects on synthesis and turnover of matrix components were compared in long-term culture. Both factors stimulated proteoglycan, collagen, noncollagen protein, and DNA synthesis in a dose-dependent manner in the range of 10–500 ng/ml. Insulin-like growth factor-I increased proteoglycan synthesis to as much as six times that of controls but was less potent than insulin-like growth factor-II. Both factors stimulated increased cell proliferation by as much as five times compared with control values, but insulin-like growth factor-I was more potent than insulin-like growth factor-II. The two factors in combination did not enhance the synthesis of matrix proteins and DNA as compared with either factor alone. Insulin-like growth factor-I counteracted the decrease in collagen synthesis and stimulated protein synthesis to a higher degree than insulin-like growth factor-II in long-term culture. Both factors had similar effects on matrix turnover, with estimated half times (t1/2) for elimination of newly labeled proteoglycans and proteins of 11 and 8 days, respectively. Insulin-like growth factor-II is capable of stimulating cell proliferation and matrix metabolism in tendon explants of young rabbits at levels similar to those of insulin-like growth factor-I; in combination, the two growth factors are unable to augment the stimulatory effects of either of the factors alone.