We examined apoptosis of eosinophils obtained from normal rats and rats infected with Trichinella spiralis (T. spiralis). A considerable percentage of normal rats underwent apoptosis within a few hours, however, the apoptosis rate of infected rats was significantly lower. This rate began decreasing only 12 h after infection, reached its lowest level on day 3, and was restored to normal by day 40. We also studied the effects on eosinophil apoptosis of culture supernatants from concanavalin A-stimulated spleen cells from infected rats and found that supernatants from days 3, 7, and 18 of infection inhibited apoptosis eosinophils from normal rats. We then examined the distribution and clearance of eosinophils in vivo. In normal rats, the largest numbers of eosinophils were found in the spleen, liver, and bone marrow. In the infected rats, however, by day 3, by which time the worms had begun to migrate, there was a striking increase in these cells in the intestines. In summary, normal eosinophils are unexpectedly sensitive to apoptosis, whereas infected eosinophils become resistant to this form of cell death.