The bacterium Lawsonia intracellularis causes the disease proliferative enteropathy. The bacterium is present in >40% of the Swedish piglet-producing herds and production losses are estimated to 2 900 SEK per sow and year. The disease is spread by faeces and the pigs are usually affected after weaning. Sick pigs may be diagnosed by e.g. PCR. To identify infected herds, antibodies can be demonstrated in blood samples, but there are indiciations that the only, commercially available method lack specificity. Since these serological analyses are cost effective and may be applied to a large number of samples, this is very unfortunate. The aim of this study is to identify a L. intracellularis-specific protein to develop a reliable diagnostic method, ELISA, for the demonstration of Lawsonia-specific antibodies in blood samples. Such a method can be very useful in surveillance of L. intracellularis at the herd level, to demonstrate the presence of or to verify freedom from the infection.