Using a microinjection method (Rokkones et al. 1985) deoxyribonucleic acid was introduced into fertilized salmonid eggs. The survival rate after a 28 day period was 91% for injected eggs in comparison to non-injected controls. A gene construct containing the mouse metallothionein promoter fused to the human growth hormone structural gene was microinjected either as a supercoiled plasmid or as a linear sequence. In Southern blot analysis of both 5 and 73 day old dissected rainbow trout embryos, as well as in 1 year old Atlantic salmon, the mouse metallothionein human growth hormone gene sequence was detected together with the chromosomal DNA when micro-injected as plasmid or as linear DNA. After digestion with Bam HI restriction endonuclease, the human growth hormone gene was excised from the high molecular weight DNA fraction. Transcription into human growth hormone specific RNA, as well as translation and release of human growth hormone immunoreactive protein, could be demonstrated in early embryonic stages.