Infections of Eimeria tenella in chicken embryos were used to compare the anticoccidial activity of ten drugs. The minimal inhibitory concentration (MIC) and minimal toxic concentration (MTC) were affected by the time of inoculation into the embryos and by the chemical nature of the compounds. Some compounds (nicarbazin, amprolium) had no effect on the development of coccidia when they were injected into embryos after the day of infection. Drugs that act early in the life cycle of coccidia (robenidine, clopidol, decoquinate, diclazuril, halofuginone, monensin, salinomycin, and lasalocid) were active at 5-125 micrograms/embryo when they were injected on the day of infection. The ionophores and halofuginone were highly toxic to embryos; most synthetic compounds were nontoxic. The incubation of merozoites in drug suspensions prior to the infection of embryos did not result in embryo toxicity, but the resultant MICs were much higher than those obtained when drugs were injected directly into the embryos. Several products were essentially inactive. Neither nicarbazin nor amprolium prevented oocyst formation. The widely divergent endpoints for the MIC and MTC of anticoccidials in embryos seriously limits the application of this technique as a screen for anticoccidial drugs.