Characteristic changes in the numbers of peripheral blood leukocytes occurred after specific challenge of chickens immunized by infection with the intracellular intestinal coccidian parasite Eimeria maxima. Within minutes of enteric or intravenous challenge with viable parasites or with soluble parasite antigen, the numbers of circulating lymphocytes and, to a lesser extent, of heterophils were reduced. This was followed by a period of leukocytosis, the main cellular constituents of which were heterophils and lymphocytes. Indirect fluorescent staining with antisera to T- or B-lymphocytes showed the depletion in lymphocytes to be accounted for mainly by a reduction in the number of T-cells. The leukopenia after oral challenge, found in immunized birds, could be transferred to normal birds by the intravenous injection of serum, plasma, extracts of leukocytes, or suspensions of viable spleen cells.