Studies on the sensitivity of recent field isolates of E. maxima to monensin

1. The sensitivity of isolates of Eimeria maxima obtained from sites where monensin had been used was compared with isolates from sites where the drug had never been used. 2. Monensin was successful in controlling infections when weight gain was used as a parameter to measure the infection and no differences were found between isolates from different sites. 3. Isolates from sites that had been exposed to monensin produced significantly greater numbers of oocysts than isolates from unexposed sites suggesting that a low level of resistance may have developed as a result of exposure to the drug.