Sera from 40 patients with a clinical diagnosis of halothane-associated hepatitis were tested for the presence of antibodies to the trifluoroacetate (TFA) halothane metabolite hapten using an ELISA assay, with TFA-albumin as the antigen. Positive results were obtained in 30% of cases of which 3/4 with encephalopathy were positive and 9/36 non-fulminant cases were positive. Antibody specificity to the TFA hapten was confirmed in each positive result by a 'hapten inhibition' experiment in which TFA albumin binding was blocked by preincubation of serum with TFA-lysine. Most probably this assay detects a relatively low affinity cross-reaction with the TFA hapten of antibodies in the patients' sera which are directed against specific TFA-labelled liver proteins. Anti-TFA-albumin antibodies were not detected in 28 normal subjects, 5 subjects with fulminant hepatic failure secondary to other causes, 6 subjects with a history of 2 or more exposures to halothane but with no evidence of liver disease and 28 patients with a variety of chronic liver diseases. It is concluded that ELISA testing using trifluoroacetylated rabbit serum albumin (TFA-RSA) as antigen is a quick and convenient assay for the confirmation of halothane-associated hepatitis in fulminant hepatic failure secondary to halothane, but is less sensitive when the illness follows a milder course.