The diagnosis of halothane hepatitis (HH) may be assisted by detection of antibodies reacting to trifluoroacetylated proteins (anti-TFA antibodies). An enzyme-linked immunosorbent assay (ELISA) utilizing trifluoroacetylated rabbit serum albumin (TFA-RSA) as antigen detected anti-TFA antibodies in 67% of sera from patients for whom a clinical diagnosis of HH was made. Anti-TFA antibodies were detected in 33% of sera when using an ELISA with liver microsomal protein from halothane-treated rabbits as antigen. Absorption of the sera with untreated rabbit liver microsomal protein before using the microsomal protein ELISA resulted in detection of anti-TFA antibodies in 42% of sera. Using the presumptive hapten N-epsilon-trifluoroacetyl-1-lysine to block antibody binding in an ELISA resulted in positive detection in 50% of sera: the results did not always agree with the other ELISA methods. The TFA-RSA ELISA was the most sensitive method and, combined with the TFA-lysine blocking ELISA, resulted in 92% of sera from HH patients testing positive for HH-associated antibodies.