Vegetative buds of Cassiopea andromeda (Cnidaria, Scyphozoa) metamorphose into polyps in the presence of short synthetic peptides containing proline as the preterminal amino acid at the carboxyl terminus. In an in vivo assay, the binding of the biologically active hexapeptide 14C-dansyl-GPGGPA in buds was shown to be specific and saturable. Scatchard plot analysis of the specific binding data revealed a dissociation constant (KD) of about 7 μM. The total number of receptors was calculated to be approximately 1×1010 bud-1 under saturation conditions. In correlation with the results of prior investigations, our finding of receptor-mediated induction of metamorphosis strongly supports the hypothesis that binding of biologically active peptides to receptors leads to initiation of phosphatidylinositol breakdown and activation of protein kinase C.