The factorial experimental design proved to be an appropriate tool for determining the effects of the independent variables on enzyme synthesis. The variable with the greatest influence over enzyme activity was the sugar source. The greatest enzyme activity was 1200 S.U., obtained with the combination of 20 g/L of glucose, 4 g/L of casein and pH at 4. When compared to glucose, lactose was considered an inferior source of carbon for the production of microbial rennin. Enzyme production kinetics was associated to and closely accompanied cell growth. Graphical analysis, together with the response surface technique, allowed the present study to achieve optimum and well-defined ranges of operation for the fermenter parameters, thereby contributing to future projects that seek to optimize the clotting power production of rennin using Mucor miehei NRRL 3420.