Streptokinase (SK) recovery from streptococcal fermentation broths by cross-flow microfiltration (CFMF) was studied. A loss of activity of 14.4% was observed, after the initial volume was concentrated eight-fold (VCF = 8.0), recovering 51.5% of the activity in the filtrate with 34.1% remaining in the retentate. Only 1% of SK activity loss could be explained by protein adsorption onto the membrane, suggesting the involvement of other factors. Immunologic experiments using polyclonal antibodies against SK have demonstrated that SK activity loss during CFMF processes could be related to the denaturation of SK, with the formation of molecules with lower (or without) activity. Accumulation of denatured SK in the retentate suggests that denaturation could be the result of aggregation and/or polymerization phenomena, leading to the formation of protein aggregates that are retained by the microfiltration membrane, thus affecting the concentration effect and the fouling phenomenon.