When the pH of the medium was maintained at the level of 6.6--6.8 and the content of KH2PO4 in it was decreased, the biosynthetic activity of Streptococcus lactis, strain MGU, became lower. The culture assimilated low quantities of phosphorus from KH2PO4 at a high concentration of this salt in the medium; presumably, the culture required large quantities of potassium ions. If the pH of the medium is maintained at a constant value, the fermentation can be stopped 21 hr after the beginning of cultivation of Str. lactis, strain MGU.