We have determined the primary structure of human LBR, an integral protein of the nuclear envelope inner membrane, and examined its interactions with lamin B and DNA. Human LBR is 68% identical to the chicken lamin B receptor and has a basic nucleoplasmic amino-terminal domain of 208 amino acids followed by a hydrophobic domain with eight putative transmembrane segments. The amino-terminal domain contains a Ser-Arg-rich stretch and consensus sites for phosphorylation by protein kinase A and p34cdc2 protein kinase. A fusion protein containing the amino-terminal domain of human LBR is recognized by autoantibodies from patients with primary biliary cirrhosis, and these serum antibodies label the nuclear envelope when examined by immunofluorescence microscopy. The LBR amino-terminal domain precipitates lamin B from nuclear extracts and retards the migration of double-stranded DNA subjected to agarose gel electrophoresis. When immobilized on nitrocellulose, the amino-terminal domain of LBR also associates with DNA, and the stretch between amino acids 71 and 100, which contains the Ser-Arg-rich stretch, is necessary for DNA binding. These results demonstrate that LBR is conserved among vertebrate species and that its nucleoplasmic domain can potentially mediate the interaction of both the nuclear lamina and the chromatin with the inner nuclear membrane.