Full-length dimers of both DNA A and DNA B of Abutilon mosaic virus have been constructed. These constructs are not infectious when inoculated into Nicotiana benthamiana as DNA; however, an infection could be obtained using the method of agroinoculation. Symptom induction required both DNA molecules but agroinoculation with DNA A alone resulted in virus spread in the majority of plants. Mutations were made in both of the open reading frames of DNA B. Coagroinoculation of wild-type DNA A and mutant B showed that DNA B is necessary for a full infection and symptom induction. Furthermore, it was shown that mutations in DNA B inhibit the ability of DNA A to spread independently. A normal infection was obtained following coagroinoculation of both DNA B mutants and wild-type DNA A. PCR analysis of DNA extracted from these plants showed that the infection was brought about by complementation rather than recombination.