Drug reactions which occurred in 3148 patients given antituberculous chemotherapy between 1953 and 1959 in 2 hospitals in South Worcestershire are noted. The occurrence of 384 reactions led to interruption of treatment in 379 patients (12.2%). Of this total, 310 (9.8%) were of the allergic and 74 (2.4%) of the toxic variety. Of those treated with streptomycin, 7.8% were affected: 7.6% of those treated with para-aminosalicylic acid (PAS), and only 0.5% of those treated with isoniazid. Toxicity to streptomycin and PAS is one of the main problems in the chemotherapy of tuberculosis today. It is desirable that all patients be under close in-patient supervision during the early weeks of treatment when the risk is greatest.