Somerson, Norman L. (National Institutes of Health, Bethesda, Md.), Paul R. Reich, Barbara E. Walls, Robert M. Chanock, and Sherman M. Weissman. Genetic differentiation by nucleic acid homology. II. Genotypic variations within two Mycoplasma species. J. Bacteriol. 92:311-317. 1966.-A deoxyribonucleic-ribonucleic acid (DNA-RNA) homology technique was used to determine genetic relatedness among the nucleic acids of eight mycoplasmas which were serologically classified as Mycoplasma hominis type 1. The DNA preparations from these organisms were each found to be distinct. No subgrouping of the M. hominis type 1 strains could be demonstrated. In contrast, when the nucleic acids from six serologically related mycoplasmas which were isolated from tissue cultures were studied, the DNA from these species could not be distinguished. The DNA buoyant densities of the tissue culture isolates were similar. These isolates were closely related genetically to a porcine mycoplasma, M. hyorhinis.