Specific immune suppression and induction of anergy in T cells are essential processes in regulation and circumvention of immune defense. IL‐10, a suppressor cytokine of T cell proliferative and cytokine responses, plays a key regulatory role in tolerizing exogenous antigens during specific immunotherapy and natural exposure. The present study demonstrates that IL‐10 induces T cell suppression by blocking the CD28 costimulatory signal. T cell receptor counting and T cell proliferation studies by anti‐CD3 and anti‐CD28 stimulation in the presence or absence of IL‐10 revealed that IL‐10 only inhibits T cells stimulated by low numbers of triggered T cell receptors and that depend on CD28 costimulation. T cells receiving a strong signal by the T cell receptor alone and that do not require CD28 costimulation are therefore not affected by IL‐10. Coprecipitation experiments demonstrated that CD28 and the IL‐10 receptor are associated in activated T cells. IL‐10 inhibited CD28 tyrosine phosphorylation, the initial step of the CD28 signaling pathway. In consequence, phosphatidylinositol 3‐kinase p85 binding to CD28 was inhibited. Thus, IL‐10‐induced selective inhibition of the CD28 costimulatory pathway demonstrates a decisive mechanism in determining whether a T cell will contribute to an immune response or become anergic.