T cell-mediated immune responses are likely to be important in the pathogenesis of systemic vasculitis. However, identifying the T cells involved has proved difficult, and there are conflicting reports regarding T cell proliferation in response to different autoantigens. Perinuclear (P) and cytoplasmic (C) anti-neutrophil cytoplasmic antibodies (ANCA) are closely associated with systemic vasculitis, and are generally specific for MPO or PR3, respectively. We studied the proliferative responses to MPO and PR3 of peripheral blood mononuclear cells from patients with P-ANCA or C-ANCA specific for these antigens by ELISA. These responses were compared with those of normal controls, and of disease controls with P- or C-ANCA not specific for MPO or PR3. The patient group as a whole showed significant T cell proliferation in response to the autoantigens compared with controls (P = 0.005). Cells from nine of 13 P-ANCA-positive, anti-MPO-positive patients proliferated in response to MPO, compared with five of 16 controls (P = 0.04). Cells from five of eight C-ANCA-positive, anti-PR3-positive patients proliferated in response to PR3, compared with two of 11 controls (P = 0.05). These experiments demonstrate that patients with P-ANCA or C-ANCA possess T cells which respond to MPO or PR3, respectively. As in other autoimmune diseases, responses to both antigens were also seen in a proportion of healthy controls. Further analysis of these responses will be important in understanding the pathogenesis of systemic vasculitis and in designing specific immunotherapy.