SETTING: Treatment of multidrug-resistant tuberculosis (MDR-TB) is often based on drug susceptibility testing (DST) results; for this reason, rapid, simple DST methods are sought which could be applied in resource-poor countries. One such method is a nitrate reductase colorimetric assay known as the Griess method. In Peru, where the incidence rate of TB is among the highest in South America, the National Institute of Health recently undertook the validation and implementation of the direct Griess method. OBJECTIVE: To describe the process of validation and implemention of the direct Griess method at the Peruvian National Institute of Health. DESIGN: Prospective study comparing the sensitivity and specificity of the direct Griess method with the Löwenstein-Jensen proportion method in determining resistance to isoniazid (INH) and rifampin (RMP) among clinical isolates. RESULTS: Among 192 specimens, the sensitivity and specificity of the Griess method for detection of INH resistance was 99.1% and 100%, respectively. For identification of RMP resistance, the sensitivity and specificity was 93.5% and 100%, respectively. CONCLUSIONS: In addition to its high sensitivity and specificity and rapid turn around time, the Griess method uses simple, inexpensive reagents and requires minimal laboratory space and technical expertise, thus providing an ideal screening tool for resource-poor settings with high rates of MDR-TB. © 2005 The Union.