Lamins A and B, the two most abundant proteins in the nuclear envelope fraction, can each be converted into disulfide cross-linked homotypic oligomers by oxidation of intrinsic disulfide bonds. The cross-linked polymers of lamin A include dimers, trimers, tetramers, and larger polymers. Those that enter acrylamide gels migrate as expected for their molecular weights. Lamin B has yielded predominantly two oligomers and the number of monomers in these oligomers has been controversial. In the present study, it is demonstrated that migration of the lamin B oligomers is sensitive to the electrophoretic conditions; thus, the oligomers have appeared to be two forms of dimer or two forms of trimer, depending upon conditions. In the present study, a third, smaller oligomer of lamin B has been detected. Because there are three oligomers of lamin B, it is possible that these are a dimer, a trimer, and a tetramer. Support for this model has been obtained by analysis of cross-linked fragments from a mild trypsin digestion of cross-linked lamina proteins. This procedure yields 62,000-, 46,000-, and 30,000-dalton fragments of lamin B. Each fragment also occurs as three homotypic oligomers. The largest oligomer, which appears to be a tetramer, is obtained in high yield under brief, mild cross-linking conditions in several cell types. These results suggest that lamin B occurs as a tetramer in the nuclear envelope.