The nerve growth factors (NGF's) isolated from cobra venom (Naja naja) and mouse submaxillary gland are closely related proteins. They are structurally similar in that about 60% of their amino acid residues are identical (Hogue-Angeletti, R. A., Frazier, W. A., Jacobs, J. W., Niall, H. D., and Bradshaw, R. A. (1976), Biochemistry, preceding paper in this issue). They are functionally similar in that they both elicit maximum neurite outgrowth from chick embryonic dorsal root ganglia at the same protein concentration. However, the extent of the response is not as great with Naja naja NGF. The cobra NGF has an affinity close to that of mouse NGF for a major proportion of the specific NGF receptors on cells from dissociated embryonic dorsal root ganglia. Despite these similarities there are differences which can be detected between the two proteins. High concentrations of Naja naja NGF will not displace approximately 20% of the binding of mouse NGF to specific NGF receptors. Moreover, Naja naja NGF shows limited cross-reactivity with antiserum to mouse NGF in a competition radioimmunoassay, consistent with the extent of its amino acid sequence homology with mouse NGF. Naja naja NGF does not interact with the α-and γ-subunits of 7S NGF to form a high molecular weight complex. In this behavior it resembles a modified form of mouse NGF which, like Naja naja NGF, lacks COOH-terminal arginine residues.