Murine mononuclear leucocytes from bone marrow, spleen, lymph node and blood stimulated in vitro by UV-irradiated herpes simplex type I virus (HSV) produced about equal proportions of IFN-alpha and -beta determined by immunoassay. Thymocytes produced only IFN-alpha. The frequency of IFN-alpha/beta mRNA containing cells detected by in situ hybridization was highest with bone marrow (15 per 10(4) cells), followed by spleen (4/10(4)), lymph node (2/10(4)), blood (1/10(4)) and thymus (0.2/10(4)). Such IFN-alpha/beta producing cells (IPCs) were heavily labelled in autoradiographs, each producing about 0.4 U of IFN. After one intravenous injection of UV-irradiated HSV in mice, high levels of IFN-alpha and -beta were present in blood at 3-9h and little or none at 24h or later. Frequent cells strongly positive for IFN-alpha mRNA at in situ hybridization and for IFN-alpha/beta at immunohistochemical staining were found almost exclusively in the marginal zones of spleens. Occasional IPCs were detected in lymph nodes but not in bone marrow, liver and kidneys. The marginal zone IPCs may be the major source of IFN in blood, and high splenic levels of IFN-alpha/beta should have efficient antiviral and immunoregulatory functions.