Summary Resistance to reinfection with Eimeria tenella is developed by the 96th hour following initial inoculation. Drug concentrations of 0.25 per cent and 0.375 per cent of Sulphamezathine in the food have a selective effect on the second generation schizonts thereby controlling mortality from caecal coccidiosis. Drug concentrations of 0.50 per cent to 1.0 per cent interfere with the normal therapeutic effect by inhibiting developmental stages previous to the second generation schizonts. In view of the well known therapeutic effect of the drug it can be assumed that Sulphamezathine is coccidiostatic to all developmental stages previous to the second generation schizonts to which it is coccidiocidal. Alternatively, it is suggested that a coccidiostatic effect on the second generation schizont is reinforced, at therapeutic levels of the drug, by the naturally developing immunity. It is concluded that the use of an excessively high concentration of the drug is not only unnecessary but may produce untoward results as instanced by the occasional failure of Sulphamezathine treatment under field conditions. The therapy must be confined, as far as possible, to the later stages of the development of the parasite.