CD30 is a type I transmembrane protein and a member of the tumor necrosis factor receptor superfamily of proteins. (1,2) Murine CD30 is expressed predominantly in the thymus, and it is inducible in mouse splenocytes stimulated with pokeweed mitogen or concanavalin A. (1) In anti-CD3epsilon-activated spleen cells, CD30 is expressed primarily on the surface of CD8+ T cells with peak expression on days 4 and 5. Stimulation of CD30+ CTL lines with plate-bound anti-CD30 directly signals IL-5 but not IFN-gamma production. (1) While these studies demonstrate that CD30 directs cytokine secretion and suggest that CD30 may play a pivotal role in the pattern of cytokine production by T cells, the precise roles of CD30 and its ligand (CD153) in T-cell development have not been clearly defined.