The human peripheral blood mononuclear cells responsible for IFN-alpha production in response to viral stimuli have been most often described as either monocytes (as typified by the response to Sendai virus) or as a light density, HLA-DR+ population which is negative for most cell surface markers characteristic of mature T cells, B cells, monocytes, or natural killer cells (as typified by the response to Herpes simplex virus (HSV)). The frequency of IFN-alpha-producing cells (IPC) responding to Sendai virus is typically 10-fold or more higher than those responding to HSV. In the current study, we have used ELISpot assays to determine the frequency of IPC responding to DNA and RNA viruses including HSV, Sendai, vesicular stomatitis virus, cytomegalovirus, adenovirus, SV40, influenza, measles, mumps, Newcastle disease virus (NDV) and human immunodeficiency virus (HIV). The enveloped viruses but not the nonenveloped viruses (adenovirus and SV40) elicited an IFN-alpha response. The frequency of IPC for each of the other viruses was more similar to the low frequency HSV-responding population than to the higher frequency Sendai virus response. These included several viruses in the same family as Sendai virus, namely the paramyxo viruses measles, mumps, and NDV. IPC were also tested for sensitivity to the lysosomotropic drug chloroquine, which diminishes IFN-alpha produced in response to HSV but not Sendai virus. With the exception of Sendai virus, chloroquine treatment abrogated the majority of IFN-alpha produced and IPC against each of the viruses. We conclude that low frequency, nonmonocytic NIPC account for the majority of IFN-alpha production in response to different viruses.