AbstractWe obtained a monoclonal antibody (MA‐1) specific for macronuclei of the ciliateParamecium caudotumand P.dubosqui.Immunoblotting showed that the antigen was a poly‐peptide of 50 kilodalton (kDa). During the process of nuclear differentiation inP. caudatum, the MA‐1 antigens appeared in the macronuclear anlagen immediately after four out of eight post zygotic nuclei differentiated morphologically into the macro‐nuclear anlagen. Afterwards, the antigens could be detected in the macronucleus through the cell cycle, and disappeared when the macronucleus began to degenerate in exconjugant cells. These results suggest that the antigens may play a role in the differentiation and function of the macronucleus. © 1992 Wiley