B lymphocyte-induced maturation protein-1 (BLIMP-1) acts during differentiation of B cells and monocytes, but was originally identified as a repressor of the IFN-beta promoter induced during viral infection. A central regulator of the intracellular response to viral infection is the interferon-inducible double-stranded RNA activated protein kinase (PKR). PKR belongs to a family of kinases that phosphorylate the eukaryotic translation initiation factor 2-alpha (eIF2alpha) and activate common downstream signaling pathways. PERK, the endoplasmic reticulum resident PKR-homologue, is activated during the unfolded protein response (UPR), a stress response involved in both macrophage activation and terminal B-cell differentiation. This suggested that BLIMP-1 might be a target of stress responses involving PERK. We demonstrate that BLIMP-1 is rapidly up-regulated during the UPR in human myeloid and B-cell lines. This response is conserved in murine B-cells and murine macrophages, in which mimics of physiological stress and classical activation stimuli also induce Blimp-1. During the UPR, BLIMP-1 mRNA is induced at the level of transcription. This response is dependent on an intact PERK signaling pathway, independent of new protein synthesis and blocked by an inhibitor of NF-kappaB. Our data provide evidence for a novel pathway linking cellular stress to BLIMP-1, a regulator of differentiation in macrophages and B cells.