From cells and culture media of embryonic rat fibroblasts (1st subculture) the acid mucopolysaccharides were isolated and fractionated. The per cent calculation of the 6 fractions was based on the content of glucuronic acid. The cultures were maintained as follows: 0.5 X 10(6) to 3 X 10(6) cells were examined in Demeter flasks at pH 7.4 or 6.6; lactate concentration was enhanced to 100 mg%. The amount of each fraction was correlated with the cell density (linear regression). The pH-value and lactate concentration in connection with cell density proved to be important factors in the modification of the MPS distribution pattern.