A procedure is described for combined immunohistochemical staining and in situ RNA-RNA hybridization of human peripheral blood mononuclear leucocytes (PBMC). These cells were first stimulated in vitro by herpes simplex virus (HSV)-infected fibroblasts and after 6 h fixed and stained with a panel of antibodies against differentiation antigens. Alpha interferon (IFN-alpha)-producing cells (IPC) were identified by in situ hybridization by means of a 35S-labelled IFN-alpha 2 cRNA probe. The IPC were infrequent, one in 200-5000 PBMC, but heavily labelled with the cRNA probe. They lacked antigens typical of T and B lymphocytes, and were also essentially negative for the Leu-M5 antigen, present on a majority of monocytes. However, 50% of IPC expressed OKM5 antigens, corresponding to the thrombospondin receptor. The IPC lacked the antigens present on null lymphocytes detected by OKT16, but most of them expressed HLA-DR, -DP and -DQ antigens. The IPC may represent a small subpopulation in the monocyte/macrophage lineage, resembling cells described as antigen presenting and stimulators of autologous mixed lymphocyte reactions. Alternatively, they constitute a subpopulation among the null lymphocytes.