The requirement for monoclonal antibodies derived from species other than rats and mice is becoming increasingly realised in veterinary, as well as human, medicine. This paper reviews current knowledge of the production of inter-species hybridomas (heterohybridomas) by the fusion of rodent myeloma cell lines with lymphocytes from species of veterinary importance. To date a number of monoclonal immunoglobulins derived from sheep, cattle, pig, rabbit, mink and primate species have been produced to a variety of different bacterial, viral and nematode pathogens as well as to blood group and MHC determinants and to hormones. The technique opens up a number of possibilities for the future; some of these applications are discussed in relation to the antibodies produced thus far.