We have observed transient elevations of serum alanine transaminase (ALT) levels in patients with aplastic anaemia who have been treated with antithymocyte globulin (ATG). Out of 18 patient episodes analysed retrospectively over a 12 month period, 15 experienced increases in ALT levels with values ranging from 1.2 to 18.5 times the upper limit of normal. In 11 of 15 episodes this was transient with ALT values returning to normal by 30 days, but in two patients this persisted for 6 months, and in a further two, until death at 34 and 145 days from unrelated causes. There was no evidence of acute viral infection or reactivation and no other drug toxicity could be implicated. We conclude that this may represent either a non-specific binding effect of ATG to hepatocytes or infection with an unidentified agent.