By using the molar ratio Hyp to Hyl, types I and II of collagen can be differentiated in 0.5 to 10 mg of dried, defatted tissue. Analyses on human skin, tendon, bone, aorta, cartilage, as well as nucleus pulposus, and annulus fibrosus of intervertebral discs are reported. Analyses of collagen of mesenchymal tissues of other vertebrates are also reported. From amino acid analysis of purified collagen samples published in the literature, the molar ratio Hyp/Hyl was calculated. Type I and type III collagen were differentiated from type II, and the latter was differentiated from type IV collagen. The molar ratios obtained with our analyses followed closely the values from previously reported amino acid analyses on purified collagen.