Methodology validation and measurement uncertainty estimation are fundamental to obtain reliable results. The microbiological methods are widely used to determine antibiotic assay, as they permit evaluation of the analyzed antibiotic activity. A microbiological assay of vancomycin was performed with adoption of experimental design 5 x 1 (interpolation in 5-point standard curve assay) with final concentrations from 6.4 to 15.6 microg/mL (standards) and 10.0 microg/mL (sample). Bacillus subtitlis (ATCC 6633) was the microorganism used, with antibiotic medium No. 8 as base layer and inoculated layer. The plates were incubated at 37 degrees C for 18 h. The method adopted for the microbiological assay of vancomycin through agar diffusion was validated according to statistic results demonstrated for suitability of the method concerning linearity, precision, and accuracy. The estimated relative expanded uncertainty (4.3%) was considered adequate for this method purpose.