Growth-blocking peptide (GBP) is an insect biogenic peptide that retards the development of lepidopteran larvae. cDNAs encoding GBP were isolated from three lepidopteran species: Pseudaletia separata, Mamestra brassicae and Spodoptera litura. Comparison of these molecules revealed that the GBP coding regions were 70% homologous. In contrast, the upstream regions of the deduced propeptides were only 33% identical. Sequence analysis further suggested that GBP shares some structural similarities with human epidermal growth factors. Bioassay data revealed that several pmol/ml of GBP stimulated DNA synthesis of a human keratinocyte cell line and of SF-9 insect cells. However, several nmol/ml of GBP did not stimulate cell proliferation at all. In vivo studies similarly indicated that low concentrations of GBP stimulated larval growth while high concentrations of GBP retarded growth. These data suggest that GBP acts as a growth factor that regulates insect larval development.