SummaryThe effect of treatment with a monoclonal antibody (MoAb) capable of neutralising interferon‐gamma (IFN‐y) on the course of coccidial infections in mice (C57BL/6 and NIH infected withEimeria vermiformisorE. pragensis, and BALBc infected withE. pragensis) was examined. The results differed with the species of parasite, the strain of mouse, the measure of infection and whether the infection was a primary or secondary one. The replication ofE. vermiformisin primary infections was enhanced in bothC57BL 6 and NIH mice, but less MoAb was required in NIH than in C57BL/6 to produce similar effects. In neither strain did treatment prevent priming or interfere with the complete immunity to challenge normally induced by moderate infection withE. vermiformis.The replication ofE. pragensisin primary infections was not affected in any of the strains of mouse but the clinical effects were exacerbated. Priming withE. pragensiswas unaffected by treatment but the partial immunity to challenge, normally induced by infection with this species, was reduced when MoAb was given 2 h before challenge. This reduction was evident as an increased faecal output of oocysts and loss of body weight. These results confirm the role of! FN‐y in resistance to coccidiosis and further emphasise the complexity of the immune response in this di