Nine strains of Streptococcus sanguis were examined for competence in genetic transformation with streptomycin resistance (str-r) as marker. Eight strains belonged to serogroup H and one to the newly-described serogroup W. Seven of the strains, one of which was the reference strain NCTC 7868 (strain Challis), were competent with str-r DNA from strain Challis. Strains NCTC 9124 (strain Wicky) and 480 were incompetent. The efficiency of transformation was examined in four different media. Use of Todd Hewitt broth gave frequencies of transformants as high as the more complex media. Addition of serum to the transformation media was not essential for the development of competence. The presence of a competence factor (CF) in the culture filtrate of strain Challis was confirmed. The factor transferred strain Wicky to competence with a great variation in the number of transformants and had no influence on strain 480. On the other hand, this spontaneously incompetent strain became competent after addition of culture filtrate from the competent strain 13b, in contrast to Wicky which now remained incompetent. Thus, it is suggested that several factors are involved in the induction of competence of S. sanguis.