If hyaline cartilage is explanted to the chick chorioallantoic membrane, it resists invasion by vascularized mesenchyme of the host. This resistance is diminished if the tissue is extracted with relatively low concentrations (1.0 M) of guanidine HCl. The extracts contain antiproteolytic activity. This molarity of guanidine HCl extracts only small amounts of the major structural components of cartilage extracellular matrix. It, therefore, seems reasonable to suggest that hyaline cartilage is both avascular and resistant to invasion because it contains extractable inhibitors of invasion, perhaps in the form of proteinase inhibitors.