The method for the determination of microbiological purity of non-sterile drugs using membrane filtration with Millipore AP15 pre-filters, prescribed by 1995 Amendment to the USSR State Pharmacopoeia XI edition, does not allow to obtain correct results. This conclusion was based on experimental data using alternative test methods. Practically all microorganisms present in the sample were retained by pre-filters. As a result main filters were falsely found to be free of microorganisms.