Flow cytometric methods for the analysis ofincorporated bromodeoxyuridine are extremely rapid andsimple. We investigated whether these methods wereuseful for detecting drugallergic hepatic injury in 18 patients with drug-allergic hepatic injury,18 healthy controls, and 9 nonallergic patientsreceiving drugs. Peripheral blood mononuclear cells werestimulated with drug solutions. Incorporation ofbromodeoxyuridine was detected after labeling with FITC, andS-phase cells were counted by flow cytometry.Percentages of S-phase cells in drug-stimulated cultureminus those in spontaneous cultures were less than 1% in both healthy controls and nonallergic patientsreceiving drugs. Taking 1% as the upper limit, 13patients (72%) were judged as positive. After the invitro addition of interleukin-2, two patients among five who had been judged as negative werejudged as positive. Lymphocyte transformation test byflow cytometry may be useful in the diagnosis ofdrug-allergic hepatic injury.