The nuclear lamina, a protein meshwork lining the nucleoplasmic surface of the inner nuclear membrane, is thought to provide a framework for organizing nuclear envelope structure and an anchoring site at the nuclear periphery for interphase chromatin. In several higher eukaryotic cells, the lamina appears to be a polymer comprised mainly of one to three immunologically related polypeptides of relative molecular mass (Mr) 60,000-75,000 (60-70K) termed lamins. Three lamins (A, B, and C) are typically present in mammalian somatic cells. Previous studies on nuclear envelopes of rat liver and Xenopus oocytes suggested that the lamina has a fibrillar or filamentous substructure. Interestingly, protein sequences recently deduced for human lamins A and C from complementary DNA clones indicate that both of these polypeptides contain a region of approximately 350 amino acids very similar in sequence to the coiled-coil alpha-helical rod domain that characterizes all intermediate-type filament (IF) proteins. Here we analyse the supramolecular organization of the native nuclear lamina and the structure and assembly properties of purified lamins, and show that the lamins constitute a previously unrecognized class of IF polypeptides.