Chemical cleaning of ultrafiltration membranes is often considered successful when the flux through a cleaned membrane is much higher than through a pristine one. Here, a novel definition of cleaning intensity is proposed as the product of the concentration of the cleaning agent and the cleaning time (Ct), and it is shown that Ct values between 0.5 and 1.0?g?h?L1 are sufficient for effective cleaning. Experiments with PES-30 and PVDF-30 membranes fouled by bovine serum albumin and cleaned with surfactant, oxidant, and formulated cleaning agents demonstrate that a good cleaning should last for 1020?min and restore the flux through a virgin membrane. More intensive cleaning increases the membrane hydrophilicity and the water flux, but soon causes more severe fouling and even membrane disintegration.