Spurious cross-reactions between plant viruses and monoclonal antibodies can be overcome by saturating ELISA plates with milk proteins.

It has been claimed recently [Dietzgen (1986) Arch Virol 91: 163-173] that a series of monoclonal antibodies (Mabs) produced against the nepovirus, arabis mosaic virus (ArMV) cross-reacted with the tobamovirus, tobacco mosaic virus (TMV). In the present report, this alleged cross-reactivity was re-examined by two ELISA procedures using Mabs produced against each of the two viruses. It was found that when highly concentrated preparations of Mabs were used, all antibodies reacted in a nonspecific manner with several plant viruses. However, when defatted milk instead of bovine serum albumin was used both as blocking agent and as diluent for the Mabs, the spurious cross-reactions between unrelated viruses were abolished. The use of milk as blocking agent did not prevent the detection of genuine cross-reactions between related nepoviruses.