ABSTRACT: The cytokine interleuken 3 (IL3) regulates the steady-state levels of transcripts of several genes in pro-B cells. A candidate gene approach (multi-probe ribonuclease protection assay) has identified tumor necrosis factor alpha (TNFa) as being specifically upregulated by IL3. (Others?) Differential display has identified the monocarboxylate lactate transporter (MCT-1) as being induced by IL3. Two of ? unidentified genes identified by differential display possess the B1 repeat region, which is suspected to play a role in regulating mRNA stability, thus raising the possibility that one way that IL3 regulates gene expression is through regulation of mRNA half-life of some target genes.