DNA extracted by a standard method from Mycoplasma hominis Sprott, resistant to 100 micrograms tetracycline, permitted the quantitative genetic transformation of tetracycline-sensitive Mycoplasma salivarium to resistance. The yield was 1 microgram DNA/10(9) cells. This DNA enabled determination of the optimum conditions for making M. Salivarium competent with CaCl2 and for studying some factors affecting transformation. Mycoplasma salivarium was transformed to resistance to 10, 20, and 30 micrograms tetracycline but not to 40 micrograms. The optimum DNA concentration for transforming resistance to 10, 20, and 30 micrograms tetracycline was the same, i.e., 50 micrograms DNA/10(8) viable cells. Treatment with DNase indicated that DNA uptake took 30 min. Competition between transforming DNA and DNA from calf thymus and M. salivarium tets inhibited transformation.