The effect of freezing on the stability of several reconstituted antibiotics packaged in disposable hypodermic syringes was studied. The following drugs were reconstituted and, after taking samples for initial assay, were frozen at -20 degrees C in Hy-Pod hypodermic syringes: cephalothin, 1 g/5 ml; cefazolin (with water), 1 g/3 ml; cefazolin (with lidocaine injection), 1 g/3 ml; cefamandole, 1 g/3.5 ml; and nafcillin, 1 g/4 ml. At three, six, and nine months, samples of the frozen solutions were allowed to thaw at room temperature and then were assayed microbiologically. All antibiotics maintained at least 90% potency at the end of nine months. Cefazolin with lidocaine, however, turned cloudy after thawing. The data suggest that cephalothin, cefazolin with water, cefamandole, and nafcillin, when frozen and stored at -20 degrees C in Hy-Pod hypodermic syringes, may be used within nine months of initial freezing.