In 9457 dairy farms located in an area with low prevalence of bovine leukemia virus (BLV) infection, bulk tank milk was examined to detect for the presence of antibodies using an ELISA test. If the result was positive or doubtful, serum of all animals in the farm was tested and bulk tank milk was tested again five times every 8-12 days. The results were used to establish decision rules in the event of a positive or doubtful result during mass screening.