Bovine (bGH) or chinook salmon (Oncorhynchus tshawytscha) growth hormone (csGH) cDNA genes were transferred by microinjection into newly fertilized northern pike (Esox lucius) eggs. Nonlethal screening of fin tissue showed genomic integration of transgenes in 88 of 1398 putative transgenic fish. Expression of bGH transgenes under transcriptional control of the Rous sarcoma virus long terminal repeat was detected in 36 of 1218 putative transgenic fish examined by radioimmunoassay of blood serum. Bovine growth hormone was also detected in mesodermal tissue of fins from microinjected fish using thin slice immunohistochemistry. Southern hybridizations of six tissues from a sample of 40 microinjected individuals revealed a high degree of mosaicism, with 30% of the fish containing detectable transgenic DNA in one or more tissues and only 41% of these containing detectable transgenes in fins. Growth of microinjected fish was quantitatively evaluated in three experiments. Average weight of microinjected fish was greater than that of controls of the same sex in four out of six groups. Significant growth enhancement (P < 0.05) was detected only for microinjected males in one experiment. Comparisons among molecular assays and individual fish growth in the founder generation indicated that the high degree of mosaicism prevented non-lethal indentification of all transgenic individuals and influences detection of growth enhancement.