We have detected a novel nuclear antigen, AF-2, which appears to be involved in cell cycle-dependent alterations of chromatin structure. Specific monoclonal antibodies detect the antigen spread over the whole cell during mitosis and in islet-like structures in the nuclei of a subpopulation of cells in interphase. Upon nucleolytic digestion of fixed cells, the antigen becomes available to the antibodies in all cells, indicating that AF-2 antigen is present during the whole cell cycle but differentially accessible. Digestion with the single strand specific S1 nuclease reveals that the alteration of chromatin structure induced by the introduction of nicks into the DNA rather than the digestion of DNA bound to the immunogenic epitope accounts for the change in accessibility of AF-2 antigen in interphase nuclei. The epitope recognized by the antibody in human cells is present in two polypeptides of 65 and 36 kDa, respectively, which are tightly bound to chromatin and cross-linkable to the nuclear matrix. The proteins also occur in the midbody during cytokinesis. The immunogenic epitope is conserved between man and fission yeast.