The language of self and nonself has had a prominent place inimmunology. This paper examines Frank Macfarlane Burnet's introductionof the language of selfhood into the science. The distinction betweenself and nonself was an integral part of Burnet's biological outlook– of his interest in the living organism in its totality, itsactivities, and interactions. We show the empirical and conceptualwork of the language of selfhood in the science. The relation betweenself and nonself tied into Burnet's ecological vision of host-parasiteinteraction. The idiom of selfhood also enabled Burnet to organizeand unify a diversity of immune phenomena. Rather than approach thelanguage of self and nonself as a bluntly imposed metaphor, we focuson its endogenous origins and immanent uses in immunology.