1. The possibility of using minute doses of the antituberculosis drug isoniazid (INH) or of its metabolites acetylisoniazid (AcINH) or isonicotinic acid (INA) as innocuous markers for monitoring patient compliance has been investigated. 2. The ingestion of these colourless and tasteless compounds can readily be demonstrated using a sensitive and specific colorimetric method for detecting INA and its metabolite isonicotinylglycine (INAG) in the urine that is rapid and simple to perform. 3. Studies on the kinetics of the urinary elimination of INA and INAG after the ingestion of 6 mg doses of either INH, AcINH or INA by small groups of volunteers indicated the potential suitability of INH or AcINH for monitoring daily or twice-daily self-medication and the appropriateness of INA as a marker for investigating the compliance of drugs prescribed for thrice-daily ingestion. 4. More extensive studies showed that over 99% of the urine samples collected within 18h of dosage with 6 mg INH would give positive results when tested for the presence of INA and INAG, and that doses of 2-6 mg INH could readily by incorporated into capsules or tablets and used as markers for monitoring the ingestion of the antituberculosis or antileprosy drugs dapsone, thiacetazone, ethionamide or prothionamide, or the antihypertensive oxprenolol. Such doses are less than a fiftieth of the normal therapeutic INH dose used in the treatment of tuberculosis. 5. Evidence is presented that INH, AcINH and INA possess most of the characteristics that one would hope to find in a marker for monitoring compliance including very limited inter-individual variability in the rates at which they are converted to the compounds being detected in the urine.