A specific, sensitive, and reliable sandwich-ELISA (enzyme-linked immunosorbent assay) has been established for the determination of choline acetyltransferase (CHAT) from porcine brain. The detection limit of the assay was 30 micrograms/l and the assay was linear up to 300 micrograms/l. The within-day and day-to-day coefficients of variation were found to be 3.3% and 4.7% respectively for low CHAT concentrations (30 micrograms/l) and 3.1% and 3.4% respectively for high levels of CHAT (300 micrograms/l). The immunoassay was more sensitive than the radiometric assay of Fonnum which is widely used for the measurement of enzyme activity. In the assay monoclonal antibody was adsorbed to the polystyrene surface of the immunoplate as the capture reagent. Using a standard peroxidase protocol the immobilized antigen was detected with a highly specific anti-CHAT antiserum raised in rabbits. Two monoclonal antibodies were available for antigen binding. One of the two--A10.29B4--reacted preferentially with the active enzyme the other one--B3.9B3--reacted only with a degraded form. The polyclonal antiserum recognized both native and denatured enzyme. The effectiveness of employing the two monoclonal antibodies separately or in combination was demonstrated by measurement of porcine CHAT diluted in human cerebrospinal fluid and serum. After some minor modifications the sandwich-ELISA could be used for the determination of CHAT from the central nervous system of the rat.