Monoclonal antibody (MAb) K 112 was generated after a single intrasplenic immunization with a recurrent laryngeal squamous-cell carcinoma. The antibody detects a 43-kDa nuclear antigen, with a pI of 5.4, which is expressed only in cycling cells. Expression is typically seen in a granular pattern excluding the nucleoli. During mitosis the bulk of the antigen is diffusely distributed in the cytoplasm. Identical reactivity was observed for tissues or cells of all mammalian species tested. These data indicate that MAb K 112 recognizes a protein belonging to the class of cell-cycle-related nuclear antigen molecules.