A salt-extractable low-molecular-weight fraction has been held responsible for the resistance of cartilage to invasion by malignant cells. To test this hypothesis, we have confronted in vitro fragments from bovine articular cartilage, from bovine nasal septum, from chick embryonic tibia, or from human knee meniscus with cells from the following malignant lines: LICR-HN-4 human squamous cell carcinoma; B16BL6 mouse melanoma; Hu-456 human transitional cell carcinoma; TE-85 and SAOS-2 human osteosarcoma; and MO4 mouse fibrosarcoma. Human embryo lung cells and chick embryonic heart cells were used as nonmalignant counterparts. Confronting cultures using living cartilage and cartilage extracted with 3 M guanidinium hydrochloride were examined light microscopically and ultrastructurally after 1 to 14 days. Neither invasion of malignant cells into the matrix of living or salt-extracted cartilage nor breakdown of collagen was observed in these cultures. In contrast, both malignant and nonmalignant cells occupied preexisting spaces in the matrix, namely, cut chondrocyte lacunae, cartilage canals, and fibrillation clefts. We concluded from these experiments that salt extraction did not alter the resistance of cartilage to invasion by malignant cells in vitro. This conclusion does not support the opinion that salt-extractable factors are solely responsible for the resistance of cartilage.