A sensitive and specific ELISA for the demonstration of antibodies to the protein p24 of enzootic bovine leukosis virus (EBVL) using a 'capture' monoclonal antibody to this protein (MAb p24) was developed. The method is sensitive enough to detect the international reference serum E4/10 in pooled blood serum samples collected from up to 50 cows, or, if a 10-fold concentrate of milk whey is tested, in samples of bulk milk collected from up to 400 cows. The application of MAb p24 has considerably increased not only the sensitivity, but also the specificity of ELISA. Moreover it is possible to differentiate reliably between positive and 'false positive' reagents by testing a suspicious sample in a pair of wells of which one is coated with MAb p24 alone and the other with the complex MAb p24 + EBLV antigen and the subsequent calculation of 'specific absorbance'. This method, showing the highest sensitivity of detection of antibodies to EBLV p24 described so far, can become an effective tool on the sanitation of infected herds as well as in checks of the EBL-free status. A diagnostic kit suitable for commercial manufacture has been devised.