Using the classical subcutaneous mouse infection of Morgenroth and Abraham, various antibiotics and a new drug, 10-undecenyl-1-yl pseudothiourea hydroiodide (AHR-1911), were studied for both, clinical and sterilizing action by local application. A lack of parallelism is evident between doses giving inhibition of lesion formation and bacteriological sterilization in the S. aureus and E. coli infection: the protective/sterilizing dose ratio varied between 1 and 53 for different drugs. In vitro, cidal/static ratios varied from 3 to 80 for S. aureus, from 2 to 4 for E. coli, and from 1 to 5 for C. albicans. In S. aureus infection, neomycin and AHR-1911 were the most effective; in E. coli infection, polymyxin B and AHR-1911, followed by neomycin. In C. albicans infection, amphotericin B was most effective, but induced irritation and abscesses at the extremely low effective dose levels (2 μg/20 g mouse). The possible implication of the cidal/static ratio in vivo effectiveness and the relativity of the values obtained in vitro comparisons according to the setup of the test are discussed