We describe a novel gene, Blimp-l (for B lymphocyteinduced maturation protein), transcripts of which are rapidly induced during the differentiation of B lymphocytes into immunoglobulin secretory cells and whose expression is characteristtc of late B and plasma cell lines. The 856 amino acid open reading frame contains five Kriippel-type zinc flnger motifs and proline-rich and acidic regions similar to those of known transcrip tion factors. Serological studies show an-100 kd protein that localizes to the nucleus. Stable or transient transfection of Blimp-7 into B cell lymphoma lines leads to the expression of many of the phenotypic changes associated with B cell differentiation into an early plasma cell stage, including Induction of J chain message and immunoglobulln secretion, up-regulation of Syndecan-7, and increased cell size and granularity. Thus, Blimp-l appears to be a pleiotropic regulatory factor capable of at least partially drfving the terminal differentiation of B cells.