The role of interleukin 2 (IL 2) in the proliferation of Theileria annulata-infected lymphoblastoid cells was studied. It was found that these cells neither require nor produce IL 2. This was based on the following findings: (a) The growth of T. annulata-infected cells was not enhanced by addition of IL 2 to the culture medium. (b) IL 2-dependent Con A-stimulated bovine peripheral blood lymphocytes (Con A-blasts) no longer required IL 2 after being infected with sporozoites of T. annulata. (c) The supernatants of T. annulata-infected cells did not support growth of IL 2-dependent Con A-blasts.