Diagnosis of drug allergy has been considered to be a multistage procedure consisting of detailed history of previous medication, analysis of symptoms, laboratory tests and provocation with the supposed sensitizing drug. Many laboratory methods are used for demonstration drug allergy, but opinions differ with regard to their specificity, sensitivity and reproducibility. Lymphocyte transformation test (LTT) is considered to be a valuable in vitro method in the diagnosis of drug allergy. For evaluation of LTT we introduced a tetrazolium-based colorimetric assay (MTT). Comparison of MTT assay with 3H-Thymidine incorporation in evaluation of Phytohaemagglutinin induced LTT resulted no significant difference in the dose-response curve. During the past 3 years 315 patients were tested with LTT-MTT method; 736 lymphocyte transformation tests mainly with antibiotics, topical anesthetics and painkillers were evaluated by MTT resulting 280 positive results. Physicians requested skin tests were performed with drugs induced no in vitro lymphocyte transformation and only 8 topical reactions without general symptoms were observed. MTT method was found to be a sensitive laboratory test in evaluation of LTT performable even in routine laboratories. Its application can lead to a substantial decrease in the number of patients who must be subjected to the possible risk of in vivo tests for the determination of the sensitizing drug.