Mutants of cauliflower mosaic virus (CaMV), generated in vitro by modification of recombinant DNA plasmids containing the viral genome, either retained the ability to induce disease symptoms on turnip plants, produced less severe symptoms or failed to induce symptoms. Wild-type symptoms were produced by a variant CaMV DNA of the Cabbage S isolate that had 4 bp in open reading frame (ORF) III replaced with a 16 bp sequence. Less severe symptoms, due to a delay in symptom appearance relative to inoculation with wild-type DNA, were induced by a mutant with a frameshift mutation in ORF II (pSA103). CaMV DNA, recovered from plants infected with pSA103, contained a second mutation which restored the original translation reading frame. Nucleic acid hybridization to 'squishes' of leaf tissue from plants that had been inoculated with mutant DNAs that included DNAs modified in each of the six major ORFs of CaMV DNA revealed that only those plants that appeared diseased had detectable CaMV nucleic acid in uninoculated leaves. Replicated CaMV DNA was also not detected in non-encapsidated and virion DNA fractions from inoculated leaves of non-diseased plants.