Bovine cells transformed by infection with the protozoan parasite Theileria annulata were inoculated subcutaneously or intraperitoneally into C.B.-17 scid mice. Mice injected subcutaneously developed solid tumours at the injection site, whilst those injected intraperitoneally developed ascites. Schizont-infected cells were found in other tissues: infected cells spread much more easily from the intraperitoneal site. Karyotyping of cells isolated from tumours showed no evidence of transfer of parasites to murine cells. These results show that the scid mouse can be used as a host for Theileria-infected bovine cells.