Abstract Strep. mutans strain BHT, a serotype b organism capable of hydrolysing arginine to ammonia and citrulline, was grown in media containing high concentrations of l-arginine (0.5–3.0 per cent, w/v) to determine the effects of ammonia production on growth rate, cell yield, viability and acid production. Ammonia production and final culture pH increased in proportion to the amount of arginine added initially. Whereas high concentrations of arginine reduced growth rate slightly, final cell yields were greatly increased. Maximum cell yields were obtained with arginine concentrations between 1.5 and 3.0 per cent, concentrations which resulted in final pH values of 7.0 and 8.0, respectively. At these concentrations, virtually 100 per cent of the cells remained viable for several days, even at 37 °C, whereas cell survival in unsupplemented media was reduced to zero within 2 days at 37 °C.