For immunohistochemical demonstration of the enkephalin octapeptide Met5-enkephalin-Arg6-Gly7-Leu8, the peptide was conjugated with a carrier protein using either glutaraldehyde or 1-ethyl-3 (3-dimethylaminopropyl)-carbodiimide as coupling agent. Antisera were raised in rabbits and their specificity was studied using the immunoblotting technique. The results suggest that glutaraldehyde selectively couples the amino terminus of the peptide to the carrier protein, while carbodiimide coupling produces a mixture of specificities. Accordingly, antiserum raised against the glutaraldehyde-induced conjugate specifically recognized the peptide carboxyl terminus and allowed immunohistochemical distinction of the octapeptide from other closely related opioid peptides, such as Leu5- and Met5-enkephalin, Met5-enkephalin-Arg6-Phe7, and Phe1-Met2-Arg3-Phe4-NH2. In contrast, antiserum raised against the carbodiimide-induced octapeptide conjugate showed a mixture of specificities. Addition of glutaraldehyde to the fixative enhanced octapeptide immunoreactivity in several tissues and revealed a previously unknown nerve system in the pituitary gland. These results support the idea that optimal immunohistochemical demonstration of small molecules, which requires conjugation to a carrier protein, is obtained when the coupling agent is included in the fixative so as to induce the actual coupling reaction during fixation.