The bacteriologic content of expressed breast milk was studied in 30 mothers at the time of expression and after 24 and 48 hours of refrigeration. There were no significant differences in colony counts between the three time intervals. All samples contained Staphylococcus epidermidis. In addition, eight other species were found, including four which were gram-negative. All samples contained less than 10(6) colonies/ml, and there were no significant differences in mean colony counts between samples expressed at home and at the hospital. We conclude that it is bacteriologically safe to refrigerate expressed breast milk for up to 48 hours.