Interleukin-16 (IL-16) is a CD8+ T cell-derived cytokine that induces chemotaxis of CD4+ T cells and CD4+ monocytes and eosinophils. Analysis by gel filtration suggests that, under physiological conditions, human IL-16 exists predominantly as a noncovalently linked multimer, but that some IL-16 may exist as a monomer. However, only the multimeric form appears to possess chemotactic activity, suggesting that receptor cross-linking may be required for activity. IL-16 also induces expression of IL-2 receptor (IL-2R) and MHC class II molecules on CD4+ T cells. Human and murine IL-16 show significant cross-species reactivity.