Unspecific binding of immunoglobulins to gastrin G cells, glucagon A cells and somatostatin D cells of the gastric mucosa or pancreas, as well as to the calcitonin-somatostatin cells of rabbit thyroid has been found to occur through a non antigen-antibody mechanism mediated at least in part by the C1q fraction of complement. The phenomenon represents a major drawback in hormone immunohistochemistry, which can be prevented by incubating the specific anti-hormone sera with anti-C1q antibodies or with complement-fixing immunocomplexes.