Sixty-one patients receiving isoniazid (INH) for chemotherapy or chemoprophylaxis were assessed by the lymphocyte-transformation test (LTT) shortly after starting treatment. Thirty-eight per cent exhibited stimulation with INH, isonicotinic acid (INA) or human-serum albumin conjugates of these haptens. In the LTT-positive group, liver dysfunction subsequently developed in 58-8%, as compared to 22-7% in the LTT-negative group (P <0.01). The difference was not accountable on the basis of age, ethnic background, sex or chemotherapy vs chemoprophylaxis. Although there was an excess of alcohol abusers in the LTT-positive group, the probability of developing liver dysfunction amongst alcohol abusers in that group was twice as great as for LTT-negative alcohol abusers. The specificity of the LTT in predicting liver damage was 83-90% (depending upon the criteria used for determining positivity), while the sensitivity of the test was only 50%.