The CD4 molecule represents a major functional T-cell surface molecule, defining an important T-cell subset, which also is expressed on monocyte, dendritic, and Langerhans cells. Various in vivo studies have demonstrated its implication in various steps of physiological T-cell activation: 1. CD4 interacts with its physiological ligand, the class II molecules, thus increasing the affinity of the conjugation between CD4+CD(3+)-TCR+ and class II+ antigen-presenting cells. 2. Through CD4, the signal transduction machinery is stimulated via its association with p56lck. In addition, CD4 has proved to be the receptor for gp120, the surface glycoprotein of HIV, that allows the virus to penetrate the CD4+ T cells and monocytes. Based on in vitro studies in various animal models, CD4 mAbs have proved to be efficient in the prevention and/or therapy of a variety of immunologically based diseases: 1. When injected early in the prodromic phase of autoimmune diseases (AID) such as diabetes, either delay or prevention is achieved with or without maintenance after therapy. 2. These mAbs have proved to be self-tolerogenic, thus allowing prolonged in vivo therapy and suppression of immunogenicity of mAb of a distinct specificity. In humans, CD4 mAbs are, or could be, used and evaluated in AID (lupus, diabetes, rheumatoid arthritis, etc.), transplantation, leukemias and lymphomas expressing CD4, and, finally, in AIDS patients, in whom CD4 mAbs can block HIV-CD4 binding and deliver a negative signal to T cell, thus blocking T-cell activation and HIV transcription. CD4 mAbs at least provide evidence that the CD4 molecules are suitable for immunomodulation and could be the target for a new pharmacological antagonist.