In terms of the crucial acid-base balance in dental plaque, the bacterial catabolism of nitrogenous compounds, such as peptides and amino acids, is of importance because the end-products can raise plaque pH. Of particular significance is the fermentation of arginine by bacteria such as Streptococcus sanguis, a numerically important plaque organism. Aspects of the uptake of this amino acid were studied and it was also shown the organism can obtain arginine from small peptides, since it possesses cell-associated exo-peptidases. Furthermore, it could grow in media containing whole protein (casein), or one of its fractions, as the sole source of organic nitrogen. The studies thus showed that S. sanguis is well equipped, in terms of endo- and exo-peptidase activities, to obtain the metabolically important arginine from whole protein. It is suggested that knowledge of this type should lead to a better understanding of overall plaque metabolism--of relevance to both cariogenic and periodontopathic plaques.