In-vitro effects of insulin-like growth factor-I (IGF-I) on superoxide production in phagocytic head kidney leucocytes (HKL) and in-vivo effects on plasma lysozyme levels were examined in the rainbow trout (Oncorhynchus mykiss). In-vitro administration of salmon IGF-I (sIGF-I), human IGF-I, and human IGF-II increased superoxide production in zymosan-stimulated HKL. A significant effect of sIGF-I on superoxide production in HKL was observed from 0.1 to 100 nM, and was equipotent to that of salmon growth hormone. IGF receptor (IGFR) type Ib was expressed in trout HKL, and also in the brain, pituitary, liver, and gills; however, the expression of another IGFR (type Ia) was not detected in HKL. In-vivo intraperitoneal injection of sIGF-I increased plasma levels of lysozyme, whereas ceruloplasmin or immunoglobulin M did not change. These results indicate that IGF-I stimulates non-specific immune functions in fish.