The feasibility of using proteinase producing fungus Humicola lutea 120-5 as a source of extracellular acid phosphatase was investigated. To enhance the acid phosphatase yield and significantly reduce the proteolytic activity the composition of casein-glucose medium containing inorganic phosphate (Pi) was modified. The regulation of phosphatase formation was controlled by Pi. The repression influence of Pi on the synthesis of phosphatase was established. A reduction of Pi (KH,PO,) concentration from 1.0 to 0.01 g/l caused approximately 5-fold increase of the phosphatase (1200 U/I) and 3-fold decrease of the proteinase (10 U/ml). The omission of Pi from the medium in which the casein (phosphoprotein) was the sore phosphatase source resulted in higher phosphatase yield (2000 U/l) and lower proteolytic activity (7.5 U/ml). Different concentrations of glucose and casein were tested to obtain the optimal medium for maximal acid phosphatase production and minimal level of proteinase. The highest acid phosphatase activity of 2500 U/l and the least amount of acid proteinase (5.5 U/ml) were achieved in 72 h shake-flask culture using Pi-free medium containing glucose and casein in concentrations of 20 and 4 g/l, respectively. The ability of the fungus H. lutea 120-5 to dephosphorylate casein providing orthophosphate for cell growth was discussed.