We have identified an immune activation gene, denoted Act-2, by differential hybridization screening of an activated T-cell library. The gene is induced rapidly after T-cell activation with phytohemagglutinin, B-cell activation with Staphylococcus aureus Cowan I, and monocyte activation with lipopolysaccharide. We have isolated a cDNA containing the full-length coding region. The deduced amino acid sequence predicts an open reading frame of 92 amino acids, including a very hydrophobic N terminus, which by weight matrix score is predicted to be a signal peptide. Using a baculovirus expression system, we have shown that this gene encodes a secreted product. It is therefore possible that Act-2 represents a newly discovered cytokine.