In industry, when membrane technology is used, cleaning of membranes is one of the most important matters both from an economical as well as from a scientific point of view. The choice of cleaning agent and cleaning conditions depends on the membranes and modules used as well as on the material fouling the membranes. Flux measurements have been used primarily when evaluating cleaning efficiency and often the fluxes have been compared to the water fluxes of the virgin membranes. However, in industry, membranes are usually cleaned before usage for the first time, so the fluxes should be compared to the cleaned membrane fluxes. In this study the results of cleaning have been studied with flux and with charge characterization. Using simultaneous streaming potential and flux measurements virgin, precleaned, fouled and cleaned membranes have been compared. The cleaning agent used was Ultrasil 10. The membranes studied were mainly polysulfone membranes, but some hydrophilized polysulfone and PVDF membranes from Dow Denmark were also used. The fouling agent was BSA. The results showed that the precleaned membranes had been modified by the detergent used. BSA fouling could be identified both by the flux as well as by the charge characterization method. Also the zeta potentials of the insufficiently cleaned membranes differed from the clean ones. The charge characterization method could also show if the fouling was inside the pores or on the top of the membrane.