The virC (formerly bak) complementation group of the nopaline-type Ti plasmid pTiC58 encodes two proteins, VirC1 and VirC2. According to the primary structure of the polypeptides predicted by the nucleotide sequence, VirC1 is composed of 231 amino acids with a total molecular mass of 25.5 kilodaltons, and VirC2 is composed of 202 amino acids with a molecular mass of 22.1 kilodaltons. The pTiC58 VirC1 and VirC2 polypeptides are equal in length to VirC1 and VirC2 of the octopine-type plasmid pTiA6NC. VirC1 proteins of pTiC58 and pTiA6NC are identical at 202 (87.4%) of the amino acid residues, and this homology is distributed fairly evenly throughout the protein. VirC2 identities occur at 142 residues (70.3%), but fall predominantly into two blocks of higher homology (84.6 and 78.5%) separated by a 41-residue segment of much lower homology (29.3%). Mutations in virC resulted in attenuated virulence on all hosts tested, the severity of attenuation varying markedly depending on the type of plant inoculated. For example, the attenuation was more pronounced on Kalanchoe than on sunflower or jimson weed. Virulence was restored to normal on all hosts by in-trans complementation with corresponding nonmutant DNA fragments of pTiC58 or of the octopine-type plasmid pTi15955. Two oligopeptides from within the predicted pTiC58 VirC1 polypeptide were synthesized and used to raise antibodies. These antibodies were used to detect the VirC1 product of both pTiC58 and pTi15955. In both cases, virC was expressed constitutively in the Agrobacterium tumefaciens ros mutant. The homology between virC genes of octopine- and nopaline-type Ti plasmids thus includes a conservation of genetic regulatory control mechanisms as well as considerable conservation of the primary structure of the protein products.