We have recently shown that a single transfusion of red blood cells to normal human volunteers significantly increases the secretion of a variety of cytokines. In the present study we explored the in vitro effect of whole red blood cells on various T cell and monocytes functions of autologous human or mouse origin. This in vitro model would allow us to further determine in future studies the membranal determinants or the intracellular products of the RBC responsible for the enhancing effect. We demonstrate in this study that addition of autologous erythrocytes to human mononuclear cells or mouse spleen cell cultures results in enhancement of cellular responses to suboptimal concentrations of mitogens. These include cell proliferation, the secretion of IL-2, colony stimulating factor (CSF), interferon-γ, tumor necrosis factor, and interleukin-6 by human MNC, and cell proliferation, IL-2, IL-3, and CSF by mouse spleen cells. The enhancing effect was dose dependent. Moreover, RBC are shown to directly enhance the expression of IL-2 receptors on both human and mouse cells without the need for the presence of mitogenic stimulation. The expression of IL-2R was measured both by acquisition of responsiveness to exogenous recombinant IL-2 and by immunofluorescence staining. We suggest that whole red blood cells exert a general enhancing effect on the secretion of a variety of cytokines and induce IL-2 receptor expression, probably through nonspecific interaction between membranal domains on erythrocytes and CD2 antigen on T cells.