In this study we demonstrate that peripheral blood mononuclear cells (PBMC) of patients with IgA nephropathy (IgAN) express mRNA for tumor necrosis factor alpha (TNF-α) at a significantly lower frequency after stimulation for 24 hours with pokeweed mitogen than do PBMC of a control population. These differences were not seen in patient samples stimulated for only 3 hours. To identify the cells responsible for TNF-α expression after overnight mitogen stimulation, we performed reverse transcriptase polymerase chain reaction analysis after preparative flow cytometry of PBMC labelled with fluorescent monoclonal antibodies. After stimulation for 24 hours, PBMC of all patients tested displayed no detectable TNF-α mRNA; in the control PBMC, TNF-α mRNA was present in CD4+ and CD8+ T cells, as well as in CD19+ B cells in some samples. These results suggest that diminished expression of TNF-α by T cells may play a role in the pathogenesis of IgAN.