We report an association of severe pulmonary hemorrhage with circulating autoantibodies to neutrophil cytoplasmic antigens (ANCA) restricted to IgM class in three patients with systemic vasculitis. ANCA were detected by indirect immunofluorescence and isotype specific solid phase radioimmunoassay (SPRIA). Institution of immunosuppressive therapy was accompanied by an isotype switch to IgG ANCA and recovery in all three patients. In an associated study, ANCA activity was found in eluates from the washed glomeruli of two postmortem cases, with the same isotype distribution as was present in the sera.