Some characteristics of immune sera after toltrazuril treatment which may be involved in the enhancement of immunity are as follows. Firstly, toltrazuril acts against all intracellular parasite stages from 1 to 168 h post-infection: intracellular schizonts of generations I, II and III and also intracellular gamonts. These damaged stages remain in the host cell for a prolonged time, during which they act as antigens which can be recognised by the immune system. Secondly, immune sera during therapeutic treatment with toltrazuril show a high antibody titre of 1:100 and 1:250 in IFAT after challenge. In the immunoblot, these sera show a strong reaction with low molecular protein fragments between 14 and 20 kDa. Toltrazuril does not interfere with the process of parasite protein recognition by the immune system during infection. Thirdly, there is a tendency for a higher IgG response after treatment with toltrazuril. Fourthly, a higher antibody titre in IFAT, immunoblot and ELISA correlates with a higher reduction in oocyst excretion, lesion scoring and increased weight gains in toltrazuril-treated animals.