This paper presents the results of a study concerning the filtration of two fermented broths, obtained through aerobic cultivation of microorganisms belonging to the actinomycete class, which produced oxytetracycline and rifamycin B, using a laboratory rotary precoat filter. The studied broths were produced by Streptomyces rimosus and Nocardia mediterranei submerged fermentations, respectively. The functional relationships between the rotation speed of the drum and the filter-aid addition per unit volume of original broth, as independent variables, and the filtration rate and expenditure of filtration, as dependent variables were elaborated using a second-order rotatable composite design. The values of the independent variables necessary to minimize the value of the expenditure of filtration and to maximize the filtration rate, were pointed out.