Interleukin-16 is secreted from a variety of immune cells as a peptide of 17 kDa which aggregates into tetrameric form essential for IL-16s direct interaction with and cross linking of its receptor, the CD4 antigen. IL-16 stimulation of CD4+ cells results in the induction of cell motility, and in addition can function as a competence growth factor for CD4+ lymphocytes. These activities suggest that IL-16 could play a role in the accumulation and activation of CD4+ cells recruited to sites of inflammation. Along those lines, IL-16 has been identified at sites of inflammation associated with several different disease states. Its function as a competence growth factor specifically for CD4+ T cells may be useful for immune reconstitution in immunodeficiency diseases such as AIDS.