Human peripheral blood mononuclear cells (PBMC) respond efficiently to viral infections with interferon (IFN)-alpha production. Using Newcastle disease virus (NDV) and Sendai virus as inducers, our experiments with purified mononuclear cell populations of healthy adult volunteers showed that monocytes were the main IFN-alpha producers in these systems. Using an ELISA, which specifically recognized IFN-alpha 2, and a bioassay, in which all subtypes were detected, IFN-alpha 2 was a major subtype expressed. Furthermore, IFN-alpha release was higher after induction with NDV than after stimulation with Sendai virus. To compare the IFN-alpha response of adult PBMC to human cord blood mononuclear cells the cells were stimulated with NDV or Sendai virus. The IFN-alpha response of cord blood mononuclear cells to Sendai virus was slightly reduced compared to that of adult cells. However, IFN-alpha 2 was a major subtype produced. In contrast, when cord blood mononuclear cells were stimulated with NDV the IFN-alpha release was strongly diminished. Furthermore, IFN-alpha 2 was not the major subtype expressed. By in situ hybridization the number of IFN-alpha-producing cells was quantified. We found that 1% of adult PBMC and newborn leukocytes showed detectable IFN-alpha mRNA after stimulation with either Sendai virus or NDV. In conclusion, these data suggest that the ability of cord blood mononuclear cells to produce IFN-alpha is diminished compared to adult PBMC. This impaired IFN-alpha response is not based on a reduced number of IFN-alpha-producing cells, but rather due to underlying control mechanisms of the newborns that are different from the regulation of adult PBMC.