Effects of growth hormone (GH) and insulin-like growth factor (IGF)-I on Escherichia coli-killing activity of murine peritoneal exudative cells (PECs) were investigated. Plasma from the mice, injected subcutaneously with saline, GH (4.8 mg/kg/day), or IGF-I(24 mg/kg/day) for 6 days, was mixed with E. coli and pooled murine PECs. Plasma from GH- and IGF-I-treated mice modestly but significantly augmented the E. coli-killing activity of PECs, as compared with that from saline controls. Plasma from IGF-I-treated mice also enhanced PEC interleukin 1 production. In the next experiment, PECs preincubated with medium, GH (10-1000 ng/mL), or IGF-I (50-5000 ng/mL) for 3 h were investigated for E. coli-killing activity. Preincubation of PECs with all concentrations of GH and IGF-I significantly enhanced the E. coli-killing activity of PECs, as compared with the medium control. These results indicate that GH and IGF-I enhance phagocytosis and the E. coli-killing activity of PECs, via a modestly increased plasma capacity to support these activities, as well as by a strong direct action.