Interleukin 10 (IL-10) has been described as a cytokine inhibitory factor downregulating IL-2 secretion and inducing T cell anergy. The data reported in this study show that preincubation of resting T cells from the human CD4+ clone SP-B21 (and clone TA-23.6) with IL-10 strongly enhances their capacity to further produce IL-2, interferon gamma (IFN-gamma), IL-4 and tumour necrosis factor alpha (TNF-alpha) after subsequent activation. In contrast, when IL-10 was added during the activation step, the previously reported specific inhibition of IL-2 synthesis was observed. Flow cytometric analysis of intracellular IL-2- and IL-4-producing cells revealed that preincubation with IL-10 increased the number of cytokine-producing cells, but did not affect their individual ability to produce these cytokines. We further show that IL-10 plays a dose-dependent role of viability maintenance factor. This effect relates to a direct anti-apoptotic effect of IL-10, which is likely independent of the expression of bcl-2, bcl-x and fas. Such paradoxal properties of IL-10 on T cells should be considered when aiming at using IL-10 as an immunosuppressive molecule in the treatment of diseases.