The toxins secreted by Ustilago maydis are encoded by dsRNA viruses. The KP6 toxin encoded by subtype P6 consists of two polypeptides alpha and beta, which are not covalently bound. Neutralizing monoclonal antibodies (MoAbs) were raised against each subunit. Some of the anti-beta MoAbs identify different epitopes in the antigen. The MoAbs were used to affinity purify ot and beta-polypeptides from culture media and to detect the precursor of the mature toxin.