In vitro cultivation of cartilage cells (chondrocytes) on biodegradable polyglycolic acid (PGA) scaffolds resulted in implants which could potentially be used to repair damaged joint cartilage or for reconstructive surgery. Cell growth kinetics were studied to define conditions under which the cellularity of implants made from isolated calf chondrocytes reached that of the parent calf cartilage. In static cultures, chondrocyte growth rates decreased as either implant thickness or implant cell density increased. Over 4 weeks of cultivation, implant permeability to glucose decreased to 3% that of-the plain polymer scaffold; this effect was attributed to the decrease in effective implant porosity associated with cartilage tissue regeneration. In a well-mixed culture, implants 1 cm in diameter by 0.3 cm thick maintained high cell growth rates over 7 weeks and had normal cell densities. Regenerated cartilage with these dimensions is large enough to resurface small joints such as the trapezium bone at the base of the human thumb. Such implants could not be grown statically, since cell growth stopped at 3-4 weeks and cell densities remained below normal. Optimization of the tissue culture environment is thus essential in order to cultivate clinically useful cartilage implants in vitro.