During analysis of autoimmune sera showing reactivity to nuclear antigens a serum producing a previously undescribed cell cycle-specific nuclear immunofluorescence pattern was observed. By indirect immunofluorescence the serum generated a densely speckled nucleoplasmic staining pattern of varying intensity with relative sparing of the nucleoli. The staining pattern could be abolished by treating cells with DNase but was not affected by RNase treatment. By flow cytometric dual parameter analysis the antigen expression was shown to increase during S phase with a maximum in G2 cells, whereas G1 and M cells expressed low levels. The cell cycle-specific expression of the target antigen was similar in some aspects to the expression of cyclin A but clearly not identical. The serum recognized a 68/60 kDa doublet in immunoblotting of soluble MOLT-4 cell extract. Antibodies eluted separately from either of the two bands produced the cell cycle-specific staining pattern as well as the doublet in immunoblotting. DNA-cellulose was used for analysis of DNA-binding property and the higher band was found to be DNA-binding, whereas the lower band was not. In summary, this serum reacts with a DNA-binding and cell cycle-specific nuclear antigen which might be a novel protein involved in cell cycle progression.