Mixed salivary proteins, obtained by fractional ultrafiltration of centrifuged human saliva were utilized by Streptococcus mutans or Streptococcus sanguis as sources of amino acids for growth, but these proteins supported only limited growth of Streptococcus salivarius or Streptococcus mitior. Comparative analyses of the isoelectric focused protein patterns before and following growth with these oral streptococci revealed a distinct pattern of salivary proteins which were susceptible to alteration by each species. Proteins with isoelectric points of 4.90, 5.33, 5.50, 5.58, 5.72, 5.80, 6.00 and 6.10 usually susceptible to alteration by S. mutans were seldom affected by the other oral streptococci. Growth attained by the oral streptococci in defined medium containing fractions of partially purified proteins was related to the presence or absence of proteins specifically altered by the organisms.