Stability of frozen rat plasma containing different antibiotics

The antibiotic activity was determined at different intervals of time on plasma samples, taken from rats treated with a certain number of commonly used antibiotics, and kept at -20 C up to 8 weeks. The results of the microbiological assays demonstrate that the stability of the antibiotics in the frozen plasma decreases in the following order: oxytetracycline > cephalexin, streptomycin, erythromycin > demeclocycline > ampicillin, amoxycillin > penicillin G, cephaloridine, rolitetracycline, and tetracycline.