Products provided in multidose containers must be adequately preserved in order to prevent contamination during repeated use. The aim of this study was to evaluate the ability of a lubricant eye drop formulation without traditional preservatives to control and prevent contamination under extreme microbial conditions. Bottles of a new lubricant eye drop with a novel preservative system (SYSTANE FREE Lubricant Eye Drops, Alcon Laboratories, Inc., Fort Worth, TX) were stored at elevated (40 degrees C) and room temperature (25 degrees C) for up to 1 year. The formulation was inoculated with microorganisms to contain high levels (approximately 1x10(6) colony forming units [cfu]/mL) of Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli, Candida albicans, and Aspergillus niger. The inoculated samples were stored at 20-25 degrees C during testing. Three types of tests were conducted: (1) tests with 1 inoculation, (2) tests with 2 inoculations over 28-day test periods, and (3) tests with 6 inoculations over a 6-month test period. Additionally, a comparison of the preservative efficacy of the SYSTANE FREE formulation to marketed products, including sodium perborate, stabilized oxychloro complex, and polyhexamethylene biguanide eye drops, was performed. In all studies, the number of survivors was determined using standard microbiological dilution pour-plate techniques. The results consistently showed that the SYSTANE FREE formulation meets preservative efficacy standards equally well, or better, than, marketed eye drop products. Even with multiple inoculations, the formulation repeatedly showed no recovery of the organisms. Though the SYSTANE FREE formulation contains no traditional preservatives, its unique blend of ions and buffers was capable of preventing contamination of the contents even after exposure to extreme conditions.