Circulating antibodies to myeloperoxidase (MPO) have been described in a variety of vasculitic syndromes, drug-induced SLE and drug-induced nephritis. We have examined the autoantibody profile in acute sera from patients with antineutrophil cytoplasmic antibody-positive vasculitis (n = 8), drug-induced nephritis (n = 4), drug-induced lupus (n = 7), SLE (n = 27) and nephritis-associated with SLE (n = 17). Significant binding to purified MPO in ELISA was given by all sera from patients with vasculitis and drug-induced nephritis but ANA sought by indirect immunofluorescence on HEp-2 cells were not detected. Both anti-MPO and ANA were found in sera from patients with drug-induced lupus. Sera from patients with SLE or SLE nephritis did not contain high titres of anti-MPO antibodies but invariably contained ANA. Anti-MPO antibodies of both IgG and IgM classes were present in all sera from patients with drug-induced disease. Although the number of samples tested was small, sera from patients with drug-induced nephritis showed significantly greater median % binding of IgM to MPO compared with drug-induced SLE. Binding to MPO by IgG in these sera was not significantly different. These findings suggest that the mechanism of interaction between hydralazine and the immune system in the two drug-induced autoimmune diseases studied may contribute to their distinct clinical features.