Three studies of drug toxicity were made in Chinese adults with pulmonary tuberculosis admitted concurrently to short-course antituberculosis regimens. The first was of streptomycin plus isoniazid plus pyrazinamide given daily (SHZ regimen), three times a week (S3H3Z3 regimen) or twice a week (S2H2Z2 regimen). The second was of pyrazinamide in the SHZ regimen and PAS in the standard daily combination of streptomycin plus isoniazid plus PAS (SPH regimen). The third was of the SHZ regimen and these 3 drugs plus rifampicin daily (SHRZ regimen). In study 1 (174 SHZ, 185 S3H3Z3, 182 S2H2Z2 patients), the incidence of arthralgia was associated with the number of doses per week (P less than 0.001). The incidence of other reactions, most of which were cutaneous or vestibular, or symptomless increases in the serum alanine transaminase (AIT) concentration, was similar on all 3 regimens. In study 2 (142 SHZ, 137 SPH patients), hepatic reactions occurred on the SHZ but not on the SPH regimen (P less than 0.002), serum AIT concentrations were distributed over a higher range on the SHZ regimen, and 2 patients had jaundice. Gastrointestinal reactions were more frequent on the SPH regimen (P = 0.06). Arthralgia was commoner on the SHZ regimen (P less than 0.05). In study 3 (38 SHZ, 41 SHRZ patients), the incidence of hepatic reactions, jaundice and arthralgia was similar in the 2 regimens. On the pyrazinamide regimens combined, hepatic reactions were marginally more frequent in patients with Australia antigen or antibody either before or during chemotherapy (P = 0.09). Serum uric acid concentrations were higher in patients on daily than on intermittent pyrazinamide (P less than 0.005), and in patients with arthralgia on the daily pyrazinamide regimen than in matched controls (P = 0.07).