Native molecular packing of types I and II collagens were compared by low-angle X-ray diffraction. Fibers from human intervertebral disc that contained different proportions of types I and II collagens were studied by X-ray diffraction, and were then analyzed biochemically to measure the constituent collagen species. Other cartilages, containing exclusively type I or type II collagen, were also examined. The equatorial diffraction established that in wet, native type II collagen, the molecules are spaced farther apart laterally than in type I collagen under the same conditions. For the disc the average lateral spacing ranged from about 14 A for a fiber from the outer annulus fibrosus containing mostly type I collagen, to 16-17 A for nucleus pulposus that contained all type II collagen. No differences were evident among dried specimens. We have also found that the meridional diffraction pattern from dry fibers of type II collagen differed from that of type I collagen. The findings indicate that under physiological conditions type II collagen fibrils contain more water than type I fibrils. Calculations suggest 50-100% more water. We propose that this difference is an inherent property of type II collagen and that it may be significant for the function of type II collagen in tissues that dissipate compressive forces. The content of glycosylated hydroxylysine residues is the chemical variable likely to be modulating fibrillar hydration.