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It has been reported (Nava, Galis, and Beiser, Nature 197:903, 1963) that immunization of rabbits with competent pneumococci stimulates the production of antibodies which inhibit transformation. Similar data reported below show that the antigenic structure of transformable streptococci in the competent state differs from the antigenic structure of cells in the noncompetent state. Competence in cultures of streptococci is due to the action of a competence-provoking protein which changes the cell surface to allow deoxyribonucleic acid (DNA) uptake (Pakula and Walczak, J. Gen. Microbiol. 31:125, 1963; Pakula and Hauschild, Can. J. Microbiol., in press). The competence-provoking protein produced by the highly transformable group H Streptococcus, strain Challis, was used to convert to competency cultures of strain Wicky, a Streptococcus of the same serological group. The latter strain does not produce measurable amounts of the enzyme and cannot, therefore, be transformed in a natural way with any significant efficiency. Competent and noncompetent cells from cultures of both strains were prepared as described before (Pakula and Walczak, J. Gen. MIicrobiol. 31:125, 1963). The cells were used to immunize rabbits for 6 weeks. Each week, three 1-ml injections were given on successive days followed by a resting period of 4 days. The rabbits were bled 10 days after the last injection. Since rabbit sera may occasionally inhibit transformation, globulin fractions were prepared as described by Strauss et al. (Proc. Soc. Exptl. Biol. Med. 105:184,1960). Transformation of the drug-sensitive recipients, Challis and Wicky, to streptomycin resistance was carried out with DNA extracted from a streptomycin-resistant mutant of strain Challis. To do this, 0.25 ml of a preparation of the competenceprovoking factor was added to 2-ml amounts of a 2.5-hr culture. After 15 miml, each tube received 0.1 ml of globulin solution. The globulins were allowed to act for 30 min. To this mixture were added 5 Mg/ml of DNA and, after 15 min, deoxyribonuclease. The cultures were then grown 110 min to allow phenotypic expression of the acquired streptomycin resistance and were assayed for the number of transformants. The inhibitory effect of globulins against competent cells on transformation is shown in Table

Inhibition of transformation of streptococci by antibodies