IL-10 is an important regulator of the production of proinflammatory cytokines. Its effect on IFN-alpha production, however, has not been reported. In this study, PBMC from healthy donors were stimulated with virus in the presence of IL-10. Human IL-10 (hIL-10) caused reductions in both the frequency of IFN-alpha-producing cells (IPC) and bulk IFN in response to herpes simplex virus type-1 (HSV-1), Sendai virus, Newcastle disease virus, and vesicular stomatitis virus. The inhibitory effect occurred when IL-10 was added 2 or 4 h before, or 2 h poststimulation with HSV or Sendai virus, but not when added 4 h postinduction. Unlike IL-10, IL-4 did not affect the IFN-alpha response to HSV. However, when PBMC were induced with Sendai virus, IFN-alpha production was also reduced by IL-4. IL-10 treatment of PBMC resulted in strong reductions in the steady state levels of both HSV- and Sendai virus-induced IFN-alpha1, -alpha2, and -beta mRNA as determined by RT-PCR. IFN-alpha production to Sendai virus occurs predominantly by monocytes, whereas most enveloped viruses stimulate low frequency "natural IFN-producing cells (NIPC)," which are thought to be dendritic cells. Peripheral blood dendritic cells were found to express the IL-10 receptor, suggesting that IL-10 may directly act on the dendritic IPC. Addition of monoclonal anti-IL-10 to PBMC resulted in a significant increase in both the frequency of IPC and the amount of secreted IFN-alpha in response to HSV but not Sendai virus. We conclude that human IL-10 can serve as both an endogenous and exogenous regulator of IFN-alpha production.