“ELISA assay to detect antibodies to Rhodococcus equi virulence associated protein A (VapA) strain in serum (clotted) samples and provides measured titres. Results will not prove active infection but will indicate the degree of exposure to and antibody production against the virulent strain. In the presence of clinical signs, a high titre may indicate the need for treatment with specific antibiotic combinations, at an early stage. Serum antibody titres < 1:160 suggest that significant exposure to the virulent form of R. equi is unlikely. Titres of 1:320 to 1:2560 suggest exposure to the virulent form of R. equi or the presence of maternally derived antibodies. Foals which have had significant exposure to the virulent form of R. equi tend to have titres of 1:5120 - 1:10240. Our assay may be used to help monitor circulating R. equi antibody levels and therefore potential passive protection following transfusion. "