In an agar plate-paper disk assay method for the antibiotic polymyxin, use of Brucella bronchiseptica NRRL B-140 in place of Escherichia coli (MacLeod strain) offered the following advantages: assay time was shortened by 6-8 hrs., zones of inhibition with more distinct edges were obtained, and no double zone phenomena were encountered in the evaluation of crude polymyxin beers.