The demonstration of vasculitis and anti-myeloperoxidase antibodies in the mercuric chloride treated Brown Norway rat provides a useful, though limited, animal model of systemic vasculitis. We describe some preliminary experiments on the effect of transfer of serum from mercuric chloride treated rats and of two forms of immunotherapy: intravenous immunoglobulin and an anti-CD4 antibody. Transfer of serum did not lead to tissue injury and neither of the two forms of therapy proved beneficial in this model.