We describe and evaluate a method to measure amitriptyline and other tricyclic antidepressants by enzyme linked immunosorbent assay, using monoclonal antibody. In this assay, biological samples were first incubated with the antibody; in a second step, free remaining antibody was allowed to bind to lysozyme-nortriptyline coated immunotitration plates. The bound fraction of the monoclonal antibody was revealed with rabbit anti-mouse serum coupled to horseradish peroxidase. The optical density of the reaction product was measured with a colorimeter at 410 nm. Specificity of the antibody was investigated by means of a Farr test showing interferences in therapeutic ranges only for chlorpromazine and phenytoine. Means of intra- and inter-assay variations were 10 and 13%, respectively. The results when compared to those obtained by gas chromatography with a selective nitrogen detector gave a correlation coefficient of 0.897. Finally, the great reliability of the monoclonal antibody, the advantages of a decreased analysis time, low cost and high capacity of the procedure contribute to make this immunoassay most suitable for clinical monitoring and pharmacokinetic studies of tricyclic antidepressants.