The broad bean strain of Tobacco mosaicirus (TMV-B) infects Nicotiana tabacum White Burley systemically whereas the tomato strain of Tomato mosaicirus (ToMV-S1) induces necrotic local lesions and is restricted to inoculated leaves. To examine the possible role of the viral movement protein (MP) in these symptom differences, a chimaeric virus (T/OMP) was produced in which the TMV-B MP gene was replaced by the ToMV-S1 MP gene. T/OMP induced the same symptoms as TMV-B in N. tabacum White Burley. However, in N. tabacum Samsun NN and other plants containing the N resistance gene, T/OMP caused necrotic lesions that were smaller than those produced by TMV-B but similar in size to those of ToMV-S1. We conclude that ToMV MP gene can substitute functionally for the TMV-B MP gene, and that the MP gene influences the size of necrotic local lesions on N-containing hosts.