Identification of CD4 on human helper/inducer T cells expressing the 60,000 M.W. surface antigen (HLA class II reactive). CD4 is present in low density on monocytes. Anti-human CD28 binds the 44 kDa MW cell surface protein on the surface of most T cells. The CD26 antigen recognizes the enzyme dipeptidyl peptidase (DPP), a serine protease. Its M.W. is 120 kDa. The CD26 antigen is associated with the binding of the TAT transactivating protein of the human immunodeficiency virus (HIV).