The polymerase chain reaction (PCR) was used to detect the expression of IFN A genes in general (with 'universal' primers) and specifically the expression of mRNA transcripts encoding the subtypes IFN-alpha-1, -alpha-2, -alpha-4, -alpha-5 and -alpha-14 (with gene specific primers) in normal human peripheral blood mononuclear cells (PBMC) and PBMC subpopulations. Our examination revealed that all transcripts tested for could be detected not only following induction with inducers such as Sendai virus, Semliki Forest virus and poly(I):poly(C), but also in the absence of induction. IFN A1, IFN A2 and IFN A4 mRNAs were found to constitute the major transcripts of Sendai virus and poly(I):poly(C) induced PBMC. Fractionation of PBMC into T cells, B cells, adherent cells, mononuclear (MN) cells and polymorphonuclear (PMN) cells revealed that these cell populations all contain specific IFN A mRNA transcripts both in the absence of an inducer and following induction with Sendai virus. The proportion of IFN A transcripts detected was dependent on the cell type investigated. IFN A1, IFN A2 and IFN A4 transcripts constituted the major RNA species present in PBMC and PMN cells. In MN cells IFN A5 transcripts were also present as a major IFN RNA species. Expression of the IFN A transcripts tested for in T cells, B cells and adherent cells did not vary significantly. These results emphasize the importance of identifying IFN A subtype expression in order to further our understanding of the biological significance of differential regulation and expression of particular IFN-alpha subtypes.