Immunohistochemistry enables the localization of pathogenic antigens in tissue sections at either the light microscopic or the electron microscopic level. Successful immunostaining with an antibody depends on a number of factors, two of the most important being the fixation of the antigen in the tissue and the visualization of the primary antibody once it has bound to the antigen. Techniques available for the visualization of bound primary antibody include the indirect-labelled immunofluorescence procedure and the unlabelled peroxidase-antiperoxidase (PAP) procedure. Direct-labelled immunohistochemistry is not now widely used but is likely to become increasingly important with the introduction of monoclonal antibodies and the development of techniques for the simultaneous localization of multiple antigens. Monoclonal antibody procedures also allow the production of antibodies against antigens which are difficult to purify such as certain transmitter markers (e.g. choline acetyltransferase) and constituents of neuronal membranes. Immunohistochemistry allows the production of detailed maps of the distribution of putative transmitters in the nervous system and in combination with tract tracing procedures is being used increasingly to identify transmitters in neuronal circuits. It has also been important in establishing the transmitter status of various neuroactive compounds in single neurones. Immunohistochemistry can be carried out on post-mortem samples and is providing information on transmitter distribution in normal and abnormal human brain.