We have tried to assess the acetylation process in tuberculosis a sample of Libyan patient, Simultaneous measurement of isoniazid (INH) and its main acetylated metabolite acetylisoniazid (AcINH) in plasma is realized by high-performance liquid chromatography (HPLC). INH and its main metabolite in human plasma were measured after taken single oral dose (200-300 mg), after 3 hours the samples of patient´s venous blood (4 ml) were collected into heparinized tubes and immediately centrifuged. The plasma samples were then deproteinized by trichloroacetic acid. The analytes separated by HPLC on pinnacle II C18 column (water). Nicotinamide is used as internal standard. The mobile phase is 0.05 M ammonium acetate buffer (pH 6) acetonitrile (99:1, v/v). The detection is by ultraviolet absorbance at 275 nm. Retention time recorded for INH, AcINH and the internal standard were 7.3, 9.9 and 14.2 min respectively. The result obtained showed that the plasma concentration of INH to 48% patient showed slow acetylation, while 52% patients are high acetylator. More than half of TB patients have defect of metabolism isoniazid.