Acid phosphatase was purified from culture filtrates of Streptomyces hygroscopicus strain JA 6599-R 27/158. Method used included as first step either ammonium sulfate precipitation or adsorption of acid phosphatase on Bentonit and the desorption of enzyme from Bentonit with alkaline buffers, adsorption to DEAE-cellulose, column chromatography on Sephadex G 50 and isoelectric focusing in Sephadex gel. The specific activity of the resulting enzyme was 51 muMol/min/mg at 25 degrees C and pH of 6.25 with p-nitrophenylphosphate as substrate. The pI detected by isoelectric focusing was at pH 7.25. The molecular weight determined by gel chromatography and by SDS electrophoresis was found to be 27 000. The pH-dependence of hydrolytic activity of acid phosphatase was substrate specific. The enzyme was found to hydrolyze essentially at pH 6.2 phosphoenolpyruvate, ATP, ADP, fructose-1,6-diphosphate and tyrosine-O-phosphate. The activity was inhibited by phosphate, molybdate, arsenate, vanadate, pyrophosphate and tetraborate. In the culture medium the acid phosphatase caused the release of phosphate from solid and soluted substrates. Therefore the involvement of acid phosphatase in the regulation of secondary metabolism was discussed.