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Background: Isoniazid and orlistat were reported to have inhibitory effect on mycobacterial ES-31 serine protease in vitro and bacterial cell growth in axenic culture., Aim: To study the cumulative effect and understand drug - drug interaction, if any, when isoniazid and orlistat used in combination., Material and Methods: Inhibition of mycobacterial ES-31 serine protease by different combinations of orlistat and isoniazid together and individually were studied using azocasein assay. Inhibition of secretion of excretory secretory ES-31 antigen in Sautan culture medium was studied under axenic condition and growth of M. tuberculosis H37Ra bacilli by CFU count on LJ-medium., Results: Orlistat and isoniazid both showed inhibitory activity of ES-31 serine protease in in vitro as well as in vivo. Individually, isoniazid showed 90% inhibition at 200 ng/ml while orlistat at 250 ng/ml showed 65% inhibition of mycobacterial ES-31 serine protease in vitro. A combination of orlistat (250 ng/ml) and isoniazid (200 ng/ml) showed 86% inhibition in vitro while 73% inhibition was observed by orlistat (25 ng/ml) and isoniazid (200 ng/ml) on bacterial growth in axenic culture., Conclusion: Significant inhibition by orlistat suggests that it could be tried in patients with intolerance to isoniazid or in those already developed isoniazid resistance. It may also be explored in the suspected TB patients as initial medication in place of antibiotics for clinical relief.