A monoclonal IgM has been characterised which recognises the nuclear lamins in all mammalian cells tested. In immunoblotting experiments using both one- and two-dimensional gels it recognises lamins A, B and C. The common antigenic determinant lies on a proteolytic fragment of 46,000 daltons which can be generated from each lamin polypeptide by treatment with chymotrypsin. In immunofluorescence experiments on whole cells and thin frozen sections, the antibody labelled only the nuclear envelope and not the nuclear interior. During mitosis, labelling was found dispersed throughout the cell cytoplasm. By immunoelectron microscopy using the antibody and protein A-gold, only the nucleoplasmic side of the nuclear envelope (the nuclear lamina) was labelled, but there was no labelling of the nuclear pores.