Polyoxin B, an antifungal agro-antibiotic, is used for treating serious plant diseases. Until now there have been no reports on the antibiotic quantification in liquid in spite of its need for fermentation studies. This work reported a microbiological assay, the cylinder-plate method, for the determination of polyoxin B. The results of assay were treated statistically by analysis of variance (ANOVA) and were found linear in the range of 10-500 mu g/ml (r(2)=0.998), precise (intra-assay: relative standard deviation (R.S.D.) = 0.64; inter-assay: R.S.D. = 1.70) and accurate. This newly established method was applied to determine the antibiotic titer in Streptomyces cacaoi fermentation with comparison to HPLC analysis. The microbiological assay was satisfactory for polyoxin B quantification, and a simple, sensitive, cost-effective and specific agar diffusion bioassay for polyoxin B was thus developed. This work also demonstrated the usefulness of the microbiological assay for quantitative analysis of antibiotics in fermentation. (C) 2008 Elsevier Ltd. All rights reserved.