The mononuclear cell surface protein IA4 was originally identified in our lab using a mAb selected because of its strong reactivity with three lymphoblastoid variant cell lines which are HLA class I deficient, are LAK susceptible, and form a high number of conjugates with LAK effectors. We previously cloned the cDNA of the IA4 protein, coding for a 267-amino-acid type III integral membrane protein, with four transmembrane domains and three possible N-glycosylation sites. The IA4 protein belongs to the tetra span transmembrane (TST) new family of surface molecules, which also includes CD9, CD37, CD53, CD63, and TAPA-1. IA4 antigen was recently recognized as belonging to a new cluster of differentiation CD82 (International CD Workshop, Boston 1993). The IA4 antigen expression pattern at the surface of immune cells from normal donors was studied. On T lymphocytes, IA4 was barely detectable on resting cells and increased 3.5- to 7-fold following PHA or PHA+PMA stimulation. This IA4 increased expression is correlated with the morphologic change in blast cells and with the expression of activation markers such as CD2 and MHC class II antigens, therefore suggesting that IA4 is an activation marker on T lymphocytes. The expression of IA4 was low on circulating resting monocytes collected by elutriation. However, these monocytes, cultured in medium alone or with GM-CSF, acquired the morphology of macrophage and simultaneously overexpressed MHC Class II, CD14, and IA4 antigens, suggesting that IA4 is a differentiation marker for macrophages, whatever the culture conditions, either adherent (plastic culture dishes) or nonadherent (Teflon culture bags). IA4 stable transfectants of the murine mastocytoma cell line P815 were obtained and used to generate a new mAb. Competitive epitope binding studies have shown that IA4 antigen presents a dominant epitope recognized by most of the mAb prepared either in our lab or elsewhere. This dominant epitope is not shared by any of the other antigens of the TST family. Using this new mAb we were able to biochemically characterize the IA4 antigen as a 28-kDa protein, highly N-glycosylated with different patterns on various cells.