Acid proteinase production using filamentous fungus Humicola lutea 120-5 was studied under batch and continuous fermentation conditions in an airlift bioreactor, A comparison with proteinase production by fungal cells, cultivated in stirred tank bioreactor was made. The process performance in both fermentation devices was similar with respect to substrate utilization, biomass, and enzyme concentration. Continuous acid proteinase production was achieved for 14 days at an optimal dilution rate of 0.05/h with maximum specific activity of 90 U/mg DW of mycelia and yield of 38 U/mg glucose. The volumetric productivity (50 U/ml . h) was approximately 3 times higher than this of the batch system. All continuous experiments were carried out without any bacterial contamination, due to the low pH (3.0-3.5) during the process. The "pellet" type growth of the fungus in the airlift reactor prevented the system from plugging with filaments. (C) 2000 Elsevier Science Inc. All rights reserved.