Immunocytochemical staining of citrulline-containing proteins, i.e. products of endogenous peptidylarginine deiminase (EC 126.96.36.199) reaction, has been performed by chemical modification of citrulline residues in situ, followed by probing with IgG specific to the modified residues. Rat spinal cords that had been preincubated in vitro to accelerate the enzyme reaction were used as samples. Not only astrocytes but also the cytoplasm of some large neurons were stained. Usefulness of the method for studying nervous tissue damage was suggested.