The effect of cloned alpha-interferon (alpha-IFN) on the in vitro and in vivo expression of HLA-ABC antigens and beta-2-microglobulin (beta-2-m) on subpopulations of human lymphoid cells was studied by flow cytometry. Mononuclear cells isolated from patients and cell cultures were labelled with saturating amounts of FITC conjugated monoclonal anti-HLA-ABC or anti-beta-2-m. Phycoerythrin conjugated monoclonal antibodies were simultaneously used for the selection of T lymphocytes. T helper lymphocytes, T suppressor lymphocytes, B lymphocytes and monocytes. In vitro, alpha-IFN induced a significant increase of beta-2-m on all subsets investigated. The increase was more pronounced on B lymphocytes (64%) and monocytes (69%) than on T lymphocytes (39%) (P less than 0.01). Also the pretreatment level of beta-2-m was found to be higher on B lymphocytes (0.64 arbitrary units (a.u.)) and monocytes (0.65 a.u.) than on T lymphocytes (0.24 a.u.) (P less than 0.001). In vivo, the expression of both HLA-ABC antigens and beta-2-m was studied in three patients 24 h after administration of 50 x 10(6) units alpha-IFN/m2 i.m. HLA-ABC antigens were significantly (P less than 0.05) increased on all subsets investigated, except for T suppressor lymphocytes. The increase in beta-2-m only reached significance on T lymphocytes. T helper lymphocytes and monocytes (P less than 0.02). At 48 h after administration of alpha-IFN, expression of HLA-ABC antigens and beta-2-m approached pretreatment levels. Enhanced expression of HLA-ABC antigens and beta-2-m could be reinduced by treatment of alpha-IFN on day 7.