A correlation between the value of the intracellular pH and the biosynthesis of fusidic acid was studied by P-31-NMR spectroscopy in two strains of the fungus Fusidium coccineum. One of the strains was highly active and the other strain had low activity with respect to the antibiotic production. The position of the orthophosphate resonance in the P-31-NMR spectra was considered as a measure of the intracellular pH. In the cells of the highly active strain pH was in the range 7.0-7.5 in the cytoplasm and 6.1-6.25 in the vacuoles. In the cells of the strain with low activity was in the range 7.3-7.9 in the cytoplasm and 6.0-6.2 in the vacuoles. During high antibiotic productivity, the intracellular pH in the highly active strain full sharply, while in the less active strain it effectively did not change. This suggested that the change in the intracellular pH was responsible for the action of the enzymes in the cells and could be a factor defining the function of the cyanide-resistant respiration pathway and consequently the synthesis of fusidic acid in F. coccineum.