The nuclear envelope lamina is a supramolecular protein assembly associated with the nucleoplasmic surface of the inner nuclear membrane, which contains three predominant polypeptide components in mammalian cells (lamins A, B and C). We previously demonstrated by immunofluorescence microscopy that the lamina is reversibly disassembled during cell division, coincident with the disassembly and reconstruction of the mitotic nuclear envelope architecture. In this paper, these immunocytochemical observations are extended with cell fractionation and immunoprecipitation studies performed on synchronized populations of tissue culture cells. With these techniques, we have established that during mitosis, lamins A and C occur in a soluble and nonmembrane-associated state. In contrast, the mitotic lamin B may be associated with membrane fragments derived from the disassembled interphase nuclear envelope. From sedimentation analysis on sucrose gradients, we have determined that all three lamins are monomeric at periods of mitotic lamina disassembly. These results, together with quantitative immunoprecipitation studies, demonstrate that the lamina is reversibly depolymerized during cell division. Attendant with the depolymerized state of the lamina, the mitotic lamins (which are phosphoproteins) have a distinctly more acidic isoelectric point and a substantially higher level of phosphorylation compared to their interphase counterparts. This indicates that reversible enzymatic phosphorylations of the lamins may be involved in modulating the state of polymerization of the lamina and its reversible mitotic disassembly.