In an effort to control the spread of caseous lymphadenitis (CLA) infection, flocks of affected sheep on six holdings were tested serologically at regular intervals using an antibody ELISA with a mean (sd) specificity of 99 (1) per cent and a sensitivity of 79 (5) per cent. Western blot assays to detect antibodies to the phospholipase D (PLD) exotoxin of Corynebacterium pseudotuberculosis were used as a further test when ELISA results were inconclusive. Owners were advised to remove from the flock any sheep that demonstrated clinical signs of CLA or tested positive for PLD by ELISA or western blot. Of the six trial flocks, one was dispersed after only two blood tests, and in another the recommendations for CLA control were not followed and infected animals were retained within the flock. In the remaining four flocks, the testing regimen and other advice enabled the disease to be controlled to such an extent that the appearance of new clinical cases of CLA was effectively halted. This remained the case for up to five years after the end of the trial. In two of these flocks, a small number of seropositive animals were detected at the last flock test). However, on the other two holdings all sheep were seronegative in the final two flock tests, consistent with the complete eradication of infection.