Three bovine cell lines and four ovine cell lines infected with Theileria parva or Theileria annulata were examined for the production ofinierferon (IFN). Biologically active IFN was detected in the tissue culture supernaiants of four of the cell lines. Only one, a bovine cell line infected with T. parva, produced IFN-y as measured by specific neutralization with a monoclonal antibody to bovine IFN-y. This observation was confirmed by analysing RNA from the cell lines on Northern blots using an IFN-y cDNA probe. The other three cell lines which produced IFN were infected with T. annulata. The IFN produced by those lines was not IFN-y.