The deformability of articular cartilage was studied in vitro in 64 bovine, canine and human joints with an indenting compressometer. Two factors contributed to the depth of the indentation: the thickness of the cartilage and its intrinsic elasticity. Within a given species, there was in general a consistent curvilinear relationship between the magnitude of the deformation and the thickness of the articular cartilage under a given level of static loading (usually 106 pounds per square inch). Although it was not possible to establish a satisfactory Young's modulus, differences in intrinsic elasticity were demonstrated by variations in the indentations produced in cartilages of comparable thickness, when subjected to the same compressive stress. These species differences corresponded roughly to variations in the water content of the articular cartilages (bovine, 87; canine, 80; human, 79%). The data are consistent with the hypothesis that deformability of articular cartilage serves to increase the congruence of joints and thereby reduce the stresses on their surfaces. A frequently postulated senescent thinning of articular cartilage received no support from measurements of 28 intact human patellas through the fifth decade of life.