An in vivo method is described showing how selective killing of macrophages in the peritoneal cavity by carrageenan affects Eimeria tenella infection in chickens. Killing of macrophages was demonstrated by the increasing loss of cytoplasmic contents under light microscopy, by the disappearance of mitochondria and endoplasmic reticulum ultrastructurally, and by the about 10%-per-hr loss of nonspecific esterase activity as uptake of carrageenan increased. Heterophils were apparently unaffected. Effect of such a selective killing on E. tenella infection seemed to be greatest when the carrageenan was injected intraperitoneally 24 hr before oocyst inoculation per os, but effect after the oocysts were inoculated was minimal or nonexistent. That chickens injected with carrageenan twice (once 24 hr before and once 48 hr after E. tenella inoculation) showed higher lesion scores with 15,000 oocysts than the controls showed with 60,000 oocysts further demonstrated the participation of macrophages in E. tenella infection.