The nuclear lamina, a protein meshwork lining the nucleoplasmic surface of the inner nuclear membrane^1,2, is thought to provide a framework for organizing nuclear envelope structure³ and an anchoring site at the nuclear periphery for interphase chromatin^3–5. 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 (M_r) 60,000–75,000 (60–70K) termed lamins^1,2. Three lamins (A, B, and C) are typically present in mammalian somatic cells. Previous studies on nuclear envelopes of rat liver⁶ and Xenopus oocytes⁷ 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^8,9 indicate that both of these polypeptides contain a region of ∼350 amino acids very similar in sequence to the coiled-coil α-helical rod domain that characterizes all intermediate-type filament (IF) proteins^10,11. 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.