Interleukin-6 (IL-6) is implicated in the in vivo proliferation of malignant plasma cells in multiple myeloma. To define the molecular basis of the IL-6-induced mitogenic response in myeloma cells, we applied STAR (subtractive transcriptional amplification of mRNA), a new differential expression analysis technology, to isolate mRNAs preferentially expressed in IL-6-treated versus untreated cultures of the factor-responsive myeloma cell line U266. From the resulting collection of STAR clones, sequence information was obtained for a total of 72 distinct transcripts. Of these, 29 were found to correspond to known genes, 22 matched expressed sequence tags in public databases and 21 showed no sequence similarity to any existing entries. Among the known genes uncovered in the screen were those encoding proteins that function in cell division, cell signalling and gene/protein expression. Northern blot analysis documented that two transcription factor genes chosen for further study, c-myc promoter-binding protein (MBP-1) and X-box binding protein 1 (XBP-1), were up-regulated in U266 cells about 3-fold relative to the cell cycle-dependent beta-actin gene 12 h after IL-6 treatment. Both genes were also similarly up-regulated by IL-6 in factor-dependent ANBL-6 myeloma cells. These results indicate that MBP-1 and XBP-1 are IL-6 genes in myeloma cells; as such, they may play a role in IL-6-mediated growth control in multiple myeloma.