The potential growth stimulating effects of the blow fly, Phaenicia sericata, on mammalian tissue were assessed by exposing human fibroblast tissue culture to maggot extracts. The growth effects of these extracts were compared to those of epidermal growth factor (EGF), recombinant interleukin 6 (IL6), and the insect hormone 20-hydroxyecdysone (EC). Results of dose-response experiments revealed that EGF had a maximum fibroblast stimulation at 66078 +/- 1979 counts per minute (cpm), with peak counts on day 6 of culture, as measured by [3H]-thymidine incorporation. P. sericata hemolymph (HL) and alimentary secretions (AS) and EC were also demonstrated to stimulate resting fibroblast tissue cultures, but the maximal stimulations only achieved 12% of EGF. Their growth rates plateaued between days 4 and 6. Addition of both HL and AS, as well as EC, significantly increased the growth rate of EGF-stimulated fibroblasts; AS increased the maximal stimulation of IL6-stimulated fibroblasts. These studies suggest the existence of intrinsic factors within the maggot which may be responsible for the growth-stimulating effects seen in maggot-infested wounds.