以圓筒平碟法， Staphylococcus aureus 6538p為實驗茵種來測定青黴素含量很低之樣品時，常產生抑制圈太小且邊緣煥散不清不易測量之現象。若能同時在標準晶溶液及檢品溶液中，加同量(約n. 10r/ml )之氯四環黴素顏酸鹽時，則可使抑制圈加大且邊緣極清晰，所得含量測定之結果亦完全正確。 The microbiological assay of antibiotics is based on the rectilinear relationship of the doses and the responses (i.e. size of the inhibition zones). In another oword, the rectilinear relationship is depends on the precision of the zone measurement. It is claimed that the precision of measurement is increased with the sharpness of zone boundaries. when the active antibiotic levels in the samples are too low (i.e. in the body fluid, blood or excretion), then zone boundaries will become diffuse. in our studies on the microbial assay of penicillin with Staphylococcus aureus 6538p as the test organism, we found an interesting phenomenon that with the addition of exactly the same quantity , about 0.10 γ/ml in the final dilution, of chlortetracycline hydrochloride (CTC. HC1) both in the standard and sample solution, we would always get a sharp-bounded inhibition zone from a low level penicillin solution. and the analytical result was as accurate as in the usual method while checked with a sample of known potency.