The invasive growth of Yoshida sarcoma and Yoshida ascites hepatoma into the rat articular cartilage was examined by light and electron microscopy.The tumor cells were inoculated into the articular spaces of Donryu strain rats and the animals were killed at various intervals ranging from one to seven days.The cells which appeared on the growing fringes of the tumors in the cartilage were mostly tumor cells, yet neutrophils, macrophages, red blood cells, chondrocytes and a closed capillary were also found. In a rare case in which invasion of the cartilage took place from the bone marrow side, osteoclasts could be seen. Phagolysosomes were prominent in the macrophages, while few of them were found in the tumor cells and other cells except for neutrophils. It was suggested that mesenchymal reaction of host induced by tumor cells played an important role in the invasive growth of malignant tumors into the cartilage.