Full-grown rabbits were injected in the knee joints with solutions of trypsin of various concentrations. The animals were sacrificed 2 weeks after the trypsin injection. Twenty-four hours before sacrifice they received 40 muCi 3H-thymidine intra-articularly. The changes in the knee joints were then studied by histological and autoradiographical methods. The injection of trypsin did not result in the development of osteoarthritis. However, autoradiography revealed that the chondrocytes started to divide after the injection. The mitosis of the chondrocytes can thus not be due to degeneration of the cartilage. The explanation put forward is that the mitosis of the chondrocytes may be the result of a decrease in the concentration of a growth controlling factor (chalone) initiated by the administration of trypsin.