B7 on antigen presenting cells is a costimulatory ligand necessary for full activation of T cell. Receptors for B7 have been known as CD28 or CTLA4. We here show that in addition to B7 mRNA, an alternatively spliced mRNA (designated as MB7-2 mRNA), that immunoglobulin (Ig)C-like domain coded by exon 3 has been spliced out, is found in activated murine splenic B cells by reverse transcriptase-polymerase chain reaction analysis. Chinese hamster ovary (CHO) cells transfected with MB7-2 bound CTLA4Ig less well than those expressing B7, but bound CD28Ig to a similar extent, indicating that IgV-like domain contains the complete binding site for CD28. In addition, IgC-like domain may participate in an increase in the affinity for CTLA4. Thus, MB7-2 represents a new form of the murine B7 with different receptor binding properties.