Background/purpose: In the absence of thorough microbiological, chemical and physical stability data, high amounts of pharmaceutical products, from which the seal has been broken, are to be discarded after preparation. We performed a generic microbiological validation study for several marketed monoclonal antibody products, in order to define conditions under which leftovers from partially extracted product can be used in order to minimize loss. Methods: From the daily practice of the Central Preparation Unit of the Netherlands Cancer Institute, used monoclonal antibody product vials were collected. To examine the integrity of the primary packaging, a VDT/S Vacuum Leak tester from Erweka was used. Vials were punctured with different types of spikes or a needle prior to experiments and examined for leakage afterward. In addition, microbiological monitoring was performed by broth simulation of the preparation method. Results: All vials (631 vials, 18 different monoclonal antibody products) showed no leakage after puncturing with a 18 G needle. However, the use of a spike system resulted in leakage in 108 of the 435 tested vials. Results from the broth simulations confirmed a higher risk of contamination after puncturing with a spike as compared to needle-punctured vials (0.5% vs. 0.05%). Conclusion: When working under aseptic preparation conditions and making use of appropriate needle, the risk of contamination is acceptably low to justify storage and reuse of leftover monoclonal antibody product from a microbiological perspective. The spikes tested lead to an unacceptably high level of loss of integrity and subsequent risk of microbiological contamination if stored in a non-classified environment. We concluded that these results could be applied generically to all monoclonal antibody products with a primary packaging composed of a glass vial and rubber stopper.