In this study microbial production of rennin from a commercial strain of Mucor miehei (NRRL 3420) has been accomplished in a continuously fed fermenter. The effects of feed D-glucose concentration (2.5-30 g dm(-3)) on milk clotting activity and on other system variables were investigated at optimum mixing and dilution rates of 400 rpm and 0.0052 h(-1), respectively, without pH control. Maximum milk clotting activity (1.24IU cm(-3)) occurred when the feed D-glucose concentration was 7.5 g dm(-3). Enzyme production continued for 500 h, producing a total milk clotting activity of 10 230 IU. At the maximum milk clotting activity point, total protein concentration, milk clotting and proteolytic activities were analysed and compared with those of a commercial rennet. The ratio of milk clotting activity to proteolytic activity, and specific milk clotting activity were determined as 1.55 x 10(-3)IU PU-1 and 5.28IU mg(-1) medium protein, respectively, denoting similar characteristics to a commercial rennet after concentration of the fermentation medium.