The object of these experiments was to study the pathogenesis and kinetics of Theileria annulata infection in the efferent lymph of the draining lymph nodes of calves. Efferent lymphatics of calves were cannulated prior to infection with T. annulata sporozoite or an allogeneic schizont cell line. Potentially lethal sporozoite challenge induced cell shut-down from days 4–6 and then a massive increase in output of blasting cells (both infected and non-infected) in the efferent lymph. The rate of lymph flow and total cell output increased to 5 to 10-fold from day 6 onwards. Sporozoites were never isolated from the efferent lymph. However, large numbers of parasite-infected cells were seen in efferent lymph from the sixth day of infection. The animals inoculated with an allogeneic T. annulata-infected cell line exhibited only a small increase in flow rate and cell output. Parasite-infected cells of recipient origin were seen in efferent lymph from day 11 onwards. However, cells of donor origin were never isolated either from efferent lymph or peripheral blood. Thus the parasite transferred from the inoculated donor cell line to the cells of the recipient before schizonts appeared in efferent lymph.