An indirect enzyme-linked immunosorbent assay (ELISA) was developed to detect antibodies to BLV in milk and serum (Juntti et al., 1989). The conjugate consists of a monoclonal anti-bovine IgG1 and IgG2 labelled with horseradish peroxidase (HRP). The indirect ELISA was calibrated with EEC reference serum E 4. Standard serum E 4 was scored positive when diluted 8192 times in negative milk and between 12,800 and 25,600 times in negative serum. The sensitivity and specificity of the indirect ELISA relative to the agar gel immunodiffusion test (AGID) were 100% and 99.8%, respectively. ELISA results for milk and sera from 614 dairy cows agreed to 100%. The absorbance value in bulk milk could be used to roughly predict the rate of BLV infection among lactating cows in a herd. An infection rate of 4 to 5% in a herd could be detected in the ELISA. Results were applied in a nation-wide screening of more than 24,000 bulk-milk samples, and the subsequent introduction of an eradication programme for BLV. The aim is to eliminate the infection from Swedish herds in 5 to 10 years.