The ability of human saliva supernatant to support the in vitro growth of Streptococcus mutans and Streptococcus sanguis 10,000). Growth of the organisms on the protein subfraction was not affected by the intrinsic proteolytic activity of saliva indicating that the proteins and not digestion products were utilized. Analysis of saliva supernatant and its subfractions for cysteine and half-cystine revealed that saliva contains very low levels of freely available cysteine, but higher levels of half-cystine, particularly in the protein subfraction.