Biotechnological processes are used for the manufacture of industrial enzymes, therapeutic proteins, antibiotics, vitamins, amino acids, polysaccharides, alkaloids, and organic acids. This chapter examines in some detail two distinct applications: the use of ultrafiltration (UF) / microfiltration (MF) in the initial clarification of an antibiotic fermentation broth, and the use of selective UF for the fractionation of protein mixtures. Antibiotic fermentations are generally performed using submerged cell cultures in which the microorganisms are grown on a solid substrate. Antibiotic recovery generally involves three main processing steps: an initial clarification of the fermentation broth, a primary isolation step, and a final purification. Most antibiotic fermentations employ fairly complex feeds to satisfy the diverse nutrient requirements needed to maintain cell viability while enhancing antibiotic production. The bioreactor conditions are often shifted after the desired cell mass is obtained so as to enhance the level of antibiotic production.