The sterility of insulin in prefilled syringes that had been prepared by visiting nurses in patients' homes and stored in their refrigerators for one month was studied. Twenty elderly diabetic patients requiring weekly home-nursing visits were enrolled in the study. At the initial study visit, a nurse filled 15 syringes with the type and amount of insulin being used by the patient. Seven syringes constituted the patient's supply for the coming week; the remaining eight syringes were appropriately labeled and placed in a separate part of the refrigerator. Upon returning to replenish the patient's insulin supply during each of the following four weeks, the nurse removed two of the stored syringes for subsequent culturing. The culture medium used was appropriate for bacteria that are usual skin flora and therefore are most likely to cause touch contamination during the syringe-filling process. Control-positive cultures were prepared by intentionally contaminating two vials of 0.9% sodium chloride injection and two vials of NPH insulin with Staph. aureus and Staph. epidermidis. Insulin from 159 syringes was tested, and no bacterial growth was detected by daily readings of cultures for one week. Of the contaminated control syringes, those containing sodium chloride injection produced positive cultures at each of the study weeks; the contaminated insulin samples, although positive at week 0, had become negative by week 3, confirming the bactericidal activity of one or more of the components of this insulin product. Prefilled insulin syringes that are prepared by nurses using good aseptic technique and are stored in the patient's refrigerator appear to remain sterile for up to one month after preparation.