Immunocytochemistry is a technique used to detect proteins within cells; immunohistochemistry is the equivalent process in tissues.

First the cells are fixed using formaldehyde to preserve structures and cross-link any proteins. Then the cells are permeabilised, to allow antibody access, using a membrane-disrupting detergent such as Triton X-100, memanol or acetone. Antibodies complementary to any proteins of interest are added and then visualised, either directly or indirectly by the addition of an enzyme- or fluorophore-conjugated secondary antibody. If using an enzyme, then it must produce an insoluble product.

This technique allows the determination of which cells express a particular protein, and where in the cell the protein is localised; however, it does not allow this study in vivo since the cell has to be killed. Fluorescent protein tagging has become more common in recent years for the study of living interactions between proteins.