Genetic transformation in Streptococcus sanguis is a complex, multi-step process, involving several factors. The competence factor (CF), occurring in the culture filtrates of some strains, is essential in streptococcal transformation, but other factors are also involved. The presence of serum during growth is not obligatory for the preparation of active culture filtrate, but serum increases the amount and duration of CF activity of some strains. The stability of the CF activity differs distinctly in filtrates prepared from various strains. This observation can be explained by the presence of a previously undescribed factor--names the competence factor inactivator (CFI)--which inactivates the CF in culture filtrates during the transforming experiment as well as during storage. The CFI is thermolabile and is estimated from the filtrates by heating (65 degrees C for 15 min). The properties of the CFI suggest that it may be a protein with enzymatic activity. The CFs from the strains Challis and 13b resist heating to 100 degrees C for 2 h, and are stable for at least 72 h if the CFI is absent or has been inactivated by heating. The CF activity in the filtrates is inactivated by pronase and chymotrypsin, suggesting that the CF may be of a polypeptide nature.