1. The pharmacokinetics of isoniazid and acetylisoniazid in plasma and saliva were compared following administration of oral and intravenous doses (200 mg) to healthy volunteers and patients. 2. In the 22 subjects studied after oral administration and the six subjects studied after intravenous administration there was complete phenotypic agreement for both slow (t1/2 greater than 130 min) and fast (t1/2 less than 130 min) acetylators using either saliva or plasma. 3. Acetylator phenotyping based on the t1/2 of INH determined using saliva collected at 2, 3, 4, 5 and 6 h after a 200 mg oral dose appears to be as reliable as that based on plasma. 4. Salivary isoniazid concentrations may provide a non-invasive alternative to plasma concentrations.