Native collagen molecules containing A and B chains were isolated from pepsin-solubilised human chorionic and amniotic membrane extracts by fractional salt precipitation and DEAE-cellulose chromatography. They exhibited a circular dichroism spectrum, and a melting curve, characteristic for a triple-helical structure. Electron microscopical investigations of their segment-long-spacing crystallites revealed a molecule similar to those of the interstitial types I, II and III collagens. After denaturation, the A and B chains were separated by DEAE-cellulose chromatography and were consistently recovered in a ratio of 1:2. Renaturation experiments indicated that only the B chains are able to reform triple-helical molecules which are stable under conditions in vivo. The data support a molecular formula A(B)2 for the native collagen molecule.