A cDNA library was prepared from the murine Th cell line Th2 D.10 and used to clone the murine homologue of Ox40 by polymerase chain reaction. Comparison of the mouse sequence with the rat revealed greater than 90% homology between the two sequences at both the DNA and protein level. Northern blot analysis found that, as in the rat, Ox40 expression appears to be restricted to activated T cells. A chimeric receptor globulin was prepared to include the mouse Ox40 extracellular domain coupled to the hinge-CH2-CH3 domains of human IgG1 (Ox40-Ig). This soluble form of the molecular was then used to identify cells bearing a ligand for Ox40. FACS analysis revealed that Ox40-Ig bound to a subset of peritoneal B cells as well as to a fraction of LPS-activated splenic B cells. Immunostaining of spleen sections using an Ag-specific conjugate and Ox40-Ig found a significant proportion of antibody-forming cells co-stained with Ox40-Ig. Immunoprecipitation of cell-surface radiolabeled peritoneal B cells suggests a specific interaction with a protein of 70 kDa.