Diabetic patients frequently use their insulin vials for prolonged periods, even though antibacterial preservatives used in multidose insulin vials are not required to be effective beyond 28 days. For this reason, we evaluated the antibacterial activity present in multidose insulin vials for up to 50 days. Multidose lente insulin vials were inoculated with S. aureus and P. aeruginosa. Vials incubated at room temperature (21°C) were sterile by 48 hours, whereas when they were incubated at refrigerator temperatures (4°C), S. aureus contamination persisted up to the 17th day and P. aeruginosa were killed after 10 days. The same vials were serially contaminated on days 17, 30, and 50, and a similar antibacterial effect was maintained. Sixty-nine multidose insulin vials used for an average of 53 days were cultured. Eight vials demonstrated bacterial contamination with 1 cfu/ml of S. epidermidis or Propionibacterium acnes. No endotoxin was detected in the multidose vials used for more than 28 days. Insulin assays on 40 multidose insulin vials used for more than 28 days showed an average insulin content of 101.6±1.9 units/ml. This study did not demonstrate significant bacterial contamination, endotoxin activity, or insulin degradation of multidose insulin in vials used for periods longer than 28 days. In addition, antibacterial preservatives were more effective at room temperature than at refrigerator temperature; thus, the practice of patients not refrigerating insulin vials is sensible.