Cetrangolo Hospital, Vicente Lopez, Argentina, 1995-1999. To describe a home-made reverse-line blot hybridisation assay for the detection of rifampicin resistance-associated mutations in the rpoB gene of Mycobacterium tuberculosis, and to evaluate the usefulness of this rifampicin oligonucleotide, or 'RIFO' assay, to predict rifampicin resistance. A total of 135 M. tuberculosis isolates from the Cetrangolo Hospital were tested using the RIFO assay, the proportion method and the Mycobacterial Growth Indicator Tube (MGIT 960). In addition, 120 drug-susceptible isolates from the Netherlands were included. The results obtained with the proportion method and the MGIT 960 system were in full agreement. In the RIFO assay, 90 of the 97 rifampicin-resistant isolates were correctly identified (sensitivity 92.8%, positive predictive value 100%). All of the drug-susceptible isolates were correctly predicted by the RIFO assay. With this home-made molecular test, rifampicin resistance in M. tuberculosis can be predicted in colonies isolated in culture in only 1 day, and can therefore shorten the laboratory turn around time for rifampicin susceptibility testing by weeks. In principle the test can also be applied directly to Zichl-Neelsen slides and clinical material, as has been demonstrated for another reverse-line blot-based assay for M. tuberculosis, spoligotyping.