An immunoassay for determining very small amounts of a particular protein in tissue samples or cells. The sample is subjected to electrophoresis on SDS-polyacrylamide gel to separate constituent proteins. The resultant protein bands are then ‘blotted’ onto a polymer sheet. A radiolabelled or fluorescently labelled antibody specific for the target protein is added; this binds to the protein, which can then be detected by autoradiography or a fluorescence detector. A variation of this technique is used to screen bacterial colonies containing cDNA clones in order to isolate those colonies expressing a particular protein. The name is derived by analogy to that of Southern blotting.