SummaryWood mice from a population showing enzootic infection with Eimeria were trapped and bred under laboratory conditions. The maintenance regime, which freed the animals from coccidia, and the breeding procedures used over several generations are described and discussed.

Transfer of Eimeria apionodes from wood mice directly into untreated laboratory mice was unsuccessful but transfer into corticosteroid-treated animals produced an oocyst output, about 1000 times less than that observed from wood mice after a similar inoculum. Repeated passage through corticosteroid-treated laboratory mice resulted in a line adapted to survival in untreated animals. This line was compared with the parent strain maintained in wood mice and some features of the oocyst output patterns, notably the pre-patent period, appeared to be controlled by the host species. The oocyst production of each population was higher in the host species to which it was adapted than in the other host species (P less than 0.001). Once adapted to laboratory mice, the line produced insignificantly different levels of oocysts in corticosteroid-treated and untreated animals (P greater than 0.05).