By analogy to processes in angiogenesis (blood vessel formation), the development of the stolonal network in colonial hydrozoa involves stimulation of branching and mutual chemotropic attraction of the growing branches by means of soluble morphogenetic factors. We have identified a glycoconjugate of about 20 kDa, termed SIF (Stolon-Inducing Factor), which induces the formation of stolon branches when applied locally. Micropipettes ejecting SIF mimic the inducing action of stolon tips, the putative sources of SIF. When whole animals are exposed to SIF, stolons sprout not only from the base of the polyps but also from abnormal sites along the entire body, even from the head. In addition, the polyp (hydranth) secretes a chitinous periderm which, in the species under investigation, normally envelops stolons but not hydranths. At high SIF doses the whole hydranth is transformed into stolon tissue. The factor has been isolated from conditioned medium and from butanol extracts of Hydractinia echinata and Podocoryne carnea.