High-pressure liquid chromatographic (HPLC) methods for the quantitative determinations of cefamandole, cefamandole nafate, and cefoxitin sodium were developed and used to study the stabilities of reconstituted solutions of the latter two antibiotics. Results from the HPLC procedures were compared with those from a previously reported colorimetric procedure. Solutions (2%) of cefamandole nafate and cefoxitin sodium in 0.9% sodium chloride injection and in 5% dextrose injection were studied at 24 degrees C and 5 degrees C. The colorimetric method, which is supposed to be specific for compounds with beta-lactam rings, gave misleading results because of the hydrolysis of the antibiotics. The results obtained by HPLC were reproducible and the procedure separated cefamandole, the hydrolyzed compound, from its ester (cefamandole nafate), which was essential for conducting these studies. The cefamandole solutions were stable for approximately five days at 24 degrees C and 44 days at 5 degrees C. The cefoxitin solutions were stable for 24 hours at 24 degrees C and for at least 13 days at 5 degrees C.