To evaluate the capacity for growth of cartilage in vitro, the cartilaginous nasal septum was cultured in a serum-free medium in short- and long-term incubation periods. The proliferative activity was assessed by relating counts per minute (cpm) obtained in the liquid scintillater after [3H]-thymidine incorporation with the DNA content of the tissue under investigation. Cell proliferation was interpreted as being synonymous with the DNA synthesizing activity or the cpm/µg DNA ratio. Short-term incubation (24 h) tended to show the same proliferative activity which we have previously found in vivo and as autotransplants, showing that the presence of serum seems to be of little importance under these conditions. Long-term incubation in a serum-free medium tended to show a gradual loss of proliferative activity, emphasizing the need for either serum or other growth-promoting factors as necessary for continued normal proliferation of cartilage in vitro