Some factors affecting the suceptibility of chick embryos to E. tenella infection have been examined. An incubation temperature of 41 °C was found to be more suitable for the development of the parasite than 38 or 39 °C. At the higher temperature development of schizonts was more rapid and mortality caused by the parasite occurred earlier and was substantially greater. Mortality, caused by E. tenella, was dose related and doses between 15000 and 45000 sporozoites produced 100 % mortality in embryos incubated at 41 °C.

There were differences in the susceptibility of three strains of W.L. embryos to E. tenella infection; ‘S’ strain W.L. embryos were more susceptible than either ‘C’ or ‘K’ strains. However, the susceptibility was not evident in hatched chickens of the ‘S’ strain.

Two laboratory strains of E. tenella (‘W’, ‘H’) were shown to produce different mortality levels in embryos; the ‘W’ strain was markedly more virulent, perhaps because schizonts of the 2nd generation matured more rapidly in both embryos and chickens inoculated with this strain of E. tenella.

I wish to thank Dr R. K. Cole of Cornell University for making three strains of W.L. embryos available to me for this work. I also wish to thank the Governing Body of the Houghton Poultry Research Station; the Agricultural Research Council and the Avian Diseases Department of Cornell University for making it possible for me to undertake a substantial part of this work at that University in 1968. Grateful thanks are due to Dr P. P. Levine, D. L'Amoreaux and M. S. Shirley for their help at various stages of the work.