Previous studies have demonstrated that various cell types can produce and secrete polyornithine-attachable neurite promoting factors when cultured. This study describes an endogenous source of polyornithine-attachable neurite promoting factors. The active material extracted from an avian smooth muscle, the expansor secundariorum, is able to enhance neurite outgrowth from embryonic chick sympathetic neurons when applied to a polyornithine substrate. Unlike other polyornithine attachable factors, the material is also able to support the neurons for at least 72 hr in the absence of any added survival factors. Partial purification of the active material was achieved by affinity chromatography on polyornithine-Sepharose. The findings support the proposal that neurite promoting factors may have a definite physiological role in addition to their well established in vitro activity.