CD80 (B7-1) is a type I transmembrane glycoprotein and a member of the immunoglobulin superfamily of cell surface receptors. It is constitutively expressed by dendritic cells, monocytes, and peritoneal macrophages. Expression of CD80 on B cells is upregulated after activation by various means including stimulation by LPS, IL-4, and surface Ig receptor cross linking with peak expression occurring between 48 and 72 hours postactivation. 1-4 The interaction of CD80/B7-1 with its ligands, CD28 and CD152/CTLA-4, provides a potent costimulatory signal for T cells activated through the CD3/T cell receptor complex.5, 6 This antibody locks binding of CTLA-4 to CD80, but it does not block stimulation of T cells by natural antigen presenting cells. 2, 7