The present invention provides an assay for detection of serum antibodies to M. paratuberculosis. A tracer, comprising a carbohydrate antigen isolated from M. paratuberculosis that is conjugated to a fluorophore, is added to a serum sample from an animal to form a mixture. The fluorescence polarization of the mixture is then measured. The presence of serum antibodies to M. paratuberculosis is indicated by a fluorescence polarization value of the mixture that is higher than the fluorescence polarization value of a control. The present invention further provides a tracer for use in a fluorescence polarization assay for antibodies specific for M. paratuberculosis. The tracer comprises a carbohydrate antigen isolated from M. paratuberculosis and conjugated to a fluorophore, such that the tracer is able to bind to antibodies specific for M. paratuberculosis to produce a detectable change in fluorescence polarization.