The intracellular development of Eimeria tenella sporozoites in in vitro cultured Madin–Darby Bovine Kidney (MDBK) cells was inhibited when parasite-infected MDBK cells were incubated with peripheral blood lymphocytes (PBL) from infected chickens. The inhibition mediated by PBL was quantified by [3H]uracil uptake and increased during the course of a series of oral infections of chickens with E. tenella. This was mirrored by the development of immunity in these birds, as assessed by counting the oocyst output following each re-infection. Similar levels of inhibition were observed using PBL from 3 inbred lines of chickens which differ in their relative susceptibility to infection with E. tenella, indicating that the genetic background of the host does not influence the production of this inhibitory activity. The inhibition could be transferred to freshly infected MDBK cells using supernatants prepared from parasite-infected monolayers incubated for 48 h with PBL from immune chickens. However, there was no inhibition using either supernatants from infected MDBK cells incubated with PBL from uninfected chickens, or supernatants from uninfected MDBK cells incubated with PBL from immune chickens. Experiments using Transwell plates showed that direct contact of PBL from immune birds with infected MDBK monolayers was not required to produce supernatants with inhibitory activity. Thus production of soluble inhibitory factor(s) by PBL from immune chickens can be specifically induced by soluble antigens present in the culture media of parasite-infected MDBK cells. These factors inhibit the intracellular development of sporozoites in in vitro culture.