A clinical M. paratuberculosis isolate was cultivated in large scale culture. A procedure for efficient preparation of a lipoarabinomannan containing antigen was developed; 25 mg of purified antigen were obtained per gram of bacterial wet weight. An ELISA based on this antigen was developed using a reference-standard-method; it had the best discriminatory efficacy when ELISA Polysorp-plates were coated with 200 ng antigen per well in hypertonic buffer (pH 6.0) at 37°C for 2 hours. Intra- and interassay variation were determined to be 20% and 27%, respectively. The ELISA was evaluated using the sera of 131 non-randomly selected animals and simultaneously culturing the ileocecal lymph nodes. After determination of the negative and positive cut-off-values positive and negative predictive values of 94% and 93% were calculated with 13 animals giving an intermediate result.