1. The sensitivity of isolates of E. maxima to methyl benzoquate, clopidol, Lerbek, Amprolmix, Pancoxin and Supacox was studied. 2. Resistance to methyl benzoquate was widespread in isolates obtained from sites where broiler chickens were reared but isolates obtained from breeder farms were sensitive to the drug. Resistance to clopidol was found in all isolates examined. Isolates resistant to both these drugs were also resistant to Lerbek but where sensitivity to one ingredient was present then Lerbek retained its efficacy. 3. All isolates were resistant to Amprolmix and Pancoxin although most were sensitive to Supacox. When given to chickens medicated with Amprolmix or Supacox isolates from breeder farms caused significantly lower weight gains than from broiler farms. 4. An attempt is made to correlate these observations with the history of medication at the sites. 5. No differences in pathogenicity were found between any of the isolates studied.