Current methods of performing antibiotic susceptibility tests on M. tuberculosis require 4-6 wk for completion and in some cases have a poor correlation with clinical results. A rapid radiometric method of sensitivity testing was developed which utilizes the incorporation of 3H-uracil into RNA as a measure of mycobacterial growth. The radiometric method is sensitive and reproducible and results are available in 48 h. An excellent correlation was found between the radiometric method and the traditional agar plate technique when the 2 were compared with respect to isoniazid, streptomycin and rifampicin sensitivities. There was no correlation between the 2 methods when ethambutol was tested. The usefulness of the radiometric method for the rapid determination of antibiotic susceptibilities of mycobacteria was confirmed.