AbstractThe phenotype and proliferation of human chondrocytes in culture were analyzed before they were implanted as autologous graft in cartilage lesions. During ten autologous chondrocyte implantations in articular cartilage lesions of the knee in ten patients, small amounts of cells to be implanted were collected and analyzed by morphology, cytochemistry (alcian blue, safranin‐O), and immunocytochemistry (antibodies anti‐S100 protein, anti‐collagen types I and II, anti‐chondroitin‐S). In four cases the cells were also cultured for 3 weeks. At 1, 10, and 20 days of culture cells were incubated with 1 µCi/ml [3H]thymidine for proliferation analysis. In all cases the cells showed the morphological appearance of mature chondrocytes, stained positively for alcian blue and safranin‐O, and revealed a strong immunoreaction for S‐100 protein, type II collagen, and chondroitin‐S but not for type I collagen. Radioisotope assay of chondrocyte proliferation at 1, 10, and 20 days of culture revealed a progressive increase in [3H]thymidine incorporation. These findings indicate that the cells before autologous implantation maintain their differentiated phenotype of mature chondrocytes and proliferate greatly. This analysis is an essential step preceding wider use of this treatment in humans. However, other biological aspects of the autologous chondrocyte graft remain to be elucidated.