Assay for polymyxin B—
Transfer an accurately weighed quantity of Ointment, equivalent to about 10,000 USP Polymyxin B Units, to a 15-mL centrifuge tube, add 10 mL of ether, stir, and centrifuge for 10 minutes. Decant and discard the clear ether. Wash the residue with 10 mL of ether, centrifuge for 10 minutes, decanting and discarding the clear ether. Wash the residue with several 10-mL portions of acetone, centrifuging, decanting, and discarding each washing until the yellow color is removed from the residue.
[NOTE—Take care not to remove any of the residue with the washings.
] Add 0.2 mL of polysorbate 80 to the residue, and mix. Transfer the mixture to a 100-mL volumetric flask with the aid of
Buffer No. 6, dilute with the same solvent to volume, and mix. Proceed as directed for polymyxin B under
Antibiotics—Microbial Assays 
81
, using an accurately measured volume of this solution diluted quantitatively with
Buffer No. 6 to yield a
Test Dilution having a concentration of polymyxin B assumed to be equal to the median dose level of the Standard.
