Several reports suggest that Agrobacterium tumefaciens nucleic acids can induce transformation of the cells of susceptible host plants and that bacteria-free tissue cultures of transformed cells contain A. tumefaciens DNA, RNA, antigens, or bacteriophages. We assayed Vinca rosea tumor DNA for base sequence homologies with A. tumefaciens DNA by DNA·DNA solution enrichment and DNA·DNA filter saturation hybridization techniques. No homologies were found by either method. The filter saturation hybridization technique included model filters containing known percentages of bacterial DNA mixed with V. rosea leaf DNA. Using this sensitive technique, we found that no more than 0.02% of the crown gall tumor genome could be homologous to A. tumefaciens DNA. This upper estimate of homology corresponds to 0.2 bacterial genome equivalent per diploid tumor cell.