Cyanogen bromide and tryptic peptides of mouse 2.5S Nerve Growth Factor (NGF) have been used to inhibit competitively binding of rabbit antiserum anti-NGF to native NGF in a solid-phase radioimmunoassay. The results showed that the larger of the two peptides obtained by cyanogen bromide cleavage (amino-acid residues 10-118) was indistinguishable from NGF in this respect, whereas the smaller N-terminal peptide (residues 1-9) was virtually non-inhibitory. Ten peptides were purified from a tryptic digest of the larger cyanogen bromide fragment. One of them, peptide G7 (residues 58-59, 60-69, 104-114, linked by disulfide bridges), was capable of almost full inhibition of binding, though only when used at a concentration about 100 times higher than that necessary to get the same effect with NGF. One other peptide, G10-H1 (residues 70-74), showed some inhibition at relatively high concentrations, but the majority of peptides, including peptide DE-5 which has been shown to be active in the NGF bioassay, were essentially non-inhibitory. A significant co-operative effect was seen when peptides G7 and G10-H1 were used in conjunction. No such effects were observed with any other combination tested.