Description: OBJECTIVE: To evaluate plasma interleukin 6 (IL-6) concentration in Standardbred racehorses by means of a novel ELISA following validation of the assay for use with equine plasma samples. SAMPLE: Plasma samples obtained from 25 Thorough breds for use in assay validation and from 319 Standardbred racehorses at rest 2 to 2.5 hours prior to warm up and racing. PROCEDURES: A sandwich ELISA was developed with equine anti--IL-6 polyclonal antibody and the biotin-streptavidin chemical interaction to enhance sensitivity. The assay was validated for specificity, sensitivity, precision, and accuracy by use of both recombinant and endogenous proteins. RESULTS: For the assay, cross-reactivity with other human and equinecy to kines was very low or absent. Serial dilution of plasma samples resulte dinproportional decreases in reactivity, in dicating high specificity of the method. Partial replacement of detection antibody with capture antibody or pre treatment of samples with capture antibody caused assay signals to significantly decrease by 55 percent. Plasma IL-6 concentration varied widely among the 319 Standard breds at rest (range, 0 to 193,630 pg/mL; mean, 6, 153 pg/mL; median, 376 pg/mL). CONCLUSIONS AND CLINICAL RELEVANCE: This ELISA method proved suitable for quantification of IL-6 concentration in equine plasma samples. Plasma IL-6 concentration was high (> 10,000 pg/mL) in 9.1