Streptokinase (SK) recovery from streptococcal fermentation broth by cross-flow microfiltration has been studied. Recovery of SK in the filtrate, independent of the volumetric concentration factor, is approximately twofold lower than the initial SK activity in the fermentation broth; moreover, the SK activity in the retentate increases during the process, reaching a concentration factor of 2.73. These results show that the membrane works more as an ultrafiltration membrane, with a rejection of S = 0.6, than as a microfiltration membrane. Under filtration conditions, the membrane permeation rate decreased with time. This decrease could be explained by deposition and interaction of material onto/with the membrane resulting in the concentration of permeable products. Studies of the individual concentration factors for the main streptococcal exocellular proteins, indicate clearly that the concentration of the proteins during the microfiltration process is independent of the size of the proteins, suggesting that other factors, such as charge and hydrophobicity, along with concentration-polarization, should be taken also into account for the understanding of this phenomenon.