At mitosis in eukaryotic cells there are profound changes of shape and structure whose causes are almost entirely obscure. What is known is that there are changes in the organization of microfilaments, including the disassembly of microfilament bundles during prophases and the accompanying rounding-up of cultured cells; the formation of transient contractile rings during cytokinesis; and, subsequently, the reassembly of microfilament bundles and the respreading of the two daughter cells. As an initial step towards the biochemical understanding of these events, in which the disassembly and reassembly of microfilaments appear to play an important part, we searched for alterations of the molecular constitution of microfilaments during mitosis. We found that non-muscle caldesmon, a protein with a relative molecular mass (Mr) of 83,000 (83K) which binds to actin and calmodulin, is dissociated from microfilaments during mitosis, apparently as a consequence of phosphorylation. This process may contribute to the changes of shape and structure of cells in mitosis, as caldesmon inhibits actomyosin ATPase.