SUMMARY Aflatoxin B1 has been found in hazardous concentrations in certain Florida commercial feeds. Therefore, research was performed to determine whether aflatoxin B1 added at the concentration of 0.2 (parts per million) (p.p.m.) to a commercial feed given to New Hampshire chickens would interfere with the efficacy of a commercial coccidiostat or Marek's disease (md) vaccine or both when New Hampshire chickens were challenge inoculated with infective Eimeria tenella oocysts or virulent md virus or both. Results indicated that previous exposure to aflatoxin B1 increased susceptibility and mortality from cecal coccidiosis in chickens but did not interfere with the protection afforded by the coccidiostat. Likewise, chickens vaccinated against md and fed aflatoxin B1 seemed more resistant against challenge exposure with md virus than did nonvaccinated control chickens. Vaccinated and nonvaccinated groups of chickens given aflatoxin B1 and subsequently exposed to cecal coccidiosis were more susceptible to challenge inoculation with MD virus than were similar groups of chickens not given aflatoxin. This conclusion was made on the basis of the severity of lesions observed.