A selective and sensitive high-performance liquid chromatographic (HPLC) method has been developed for the measurement of colistin residues in milk and in four bovine tissues (i.e., muscle, liver, kidney, and fat). The sample treatment consists of protein precipitation using 10% (w/v) trichloroacetic acid, solid-phase purification on C18 cartridges, and precolumn derivatization of colistin with ortho-phthalaldehyde and 2-mercaptoethanol in borate buffer (pH 10.5). This latter step is performed automatically, and the resulting reaction mixture is injected into a switching HPLC system including a precolumn and an analytical column packed with end-capped LiChrospher RP18 (5 microns). Washing the precolumn and final elution onto the analytical column are conducted using acetonitrile-0.01M phosphate buffer (pH 7.0) mixtures with respective proportions of 65:35 and 68:32 (v/v). Detection is carried out by spectrofluorometry (excitation wavelength, 340 nm; emission wavelength, 440 nm). The retention times of the derivatives corresponding to the two main components of colistin (i.e., polymyxins E2 and E1) are approximately 14 and 18 min, respectively. The structural study of the derivatives corresponding to polymyxins E1 and E2 is carried out by HPLC coupled with electrospray mass spectrometry; data obtained confirms that the derivatization process occurs with the five amino groups of the analytes. Selectivity is obtained in the HPLC system versus other coadministered anti-infective drugs (beta-lactams, aminoglycosides, tetracyclines, and sulphonamides) and endogenous compounds. Quantitation is performed using the sum of the peak areas of polymyxin E1 and polymyxin E2 derivatives. Testing linearity affords correlation coefficients greater than 0.990 for calibration curves in the range of 10-500 microL/L for milk, 50-1000 micrograms/kg for muscle and fat, and 100-1000 micrograms/kg for kidney and liver. Relative standard deviation values are less than 10% at a concentration of 25 micrograms/L in milk and 100 micrograms/kg in tissues (six replicates); recoveries are higher than 60%.