Maturing rainbow trout were immunised with testis plus Freund's complete adjuvant (FCA) to determine when autoantigen(s1 appear on germ cells during spermatogenesis. Only testes containing spermatozoa and spermatids developed lesions, and no other germ cell types appeared to be involved. This response was gonad specific, but showed little correlation with antibody titres to spermatozoa. The necessary conditions that allow the development of autoantigen(s1 on postmeiotic cells are discussed.