Eflornithine [2-(difluoromethyl)-o~-ornithine monohydrochloride, monohydrate; DFMO] is an ornithine decarboxylase inhibitor used to treat Pneumocystis carinii pneumonia (PCP) in AIDS patients unresponsive or intolerant to conventional drug therapy. A reversedphase HPLC method utilizing precolumn dansylation is described which permits the analysis of DFMO in serum of AIDS patients in support of pharmacokinetic studies. Norvaline (2-amino valeric acid) was added as the internal standard and the sample was extracted with three portions of ice-cold 80% ethanol. The supernatants were evaporated in a vacuum oven at 50 "C, and the residue was dissolved in base and derivatized at 25 "C for 4 h in the dark using dansyl chloride in acetone. Dilute base was then added and 25 to 50 pL was injected into the HPLC. Chromatography was performed on a C-8, 15-cm column with a linear gradient from a 955 solution of 10 mM pH 4 acetate buffer and THF, to a 9O:lO solution of acetonitrile and THF over a period of 28 rnin. Detection was by UV at 330 nm, and DFMO and norvaline eluted at 14.7 and 17.7 rnin, respectively. The method was linear over a range of 5 to 950 pg/ mL, and replicate analyses of a 25-pg/mL control specimen produced a