AbstractThe healing of articular surface defects has been studied with conventional histology, which relies on the staining of the extracellular matrix to identify the phenotype of the cells present. A chondrospecific cellular marker would be useful. S‐100 protein has been found in all chondroid tissues studied, and we evaluated its usefulness in the study of articular cartilage repair. Fullthickness rabbit femoral condylar defects were made, and the specimens were studied at serial time intervals. S‐100 protein staining positively showed chondroid cells in the 7‐ and 14‐day specimens, which were not identifiable by conventional techniques. At 30 and 60 days, an S‐100 positive band of cells separated a deep safranin‐O positive hypertrophic layer from a fibrocellular surface layer. At 120 days, the presence of S‐100 protein identified cells with chondrogenic potential, and the lack of S‐100 protein in other cells embedded in conventionally stained matrix suggested that these cells were no longer of a chondroid phenotype. The presence of S‐100 protein‐identified chondroid cells early in the repair process when the cells had not begun to synthesize conventionally stainable matrix and the lack of S‐100 protein in cells late in the repair positively identified a phenotypic change earlier than c