Proteins from nine normal rat organs, including heart, lung thymus, liver, spleen, kidney, prostate, abdominal muscle, and brain, were solubilized, separated by electrophoresis according to their different isoelectric points and molecular weights, and stained with Coomassie blue. Patterns of major proteins unique to each organ were identified. Nine "unknown" samples, chosen from the same sources and submitted for analysis in a single blind study, were easily identified by comparing their protein profiles against the nine "reference" patterns. The ability to identify the origin of a tissue sample without recourse to microscopy, by comparing the pattern of its electrophoresed proteins with a "catalogue" of identified protein profiles, provides a prototype for the identification of histologically indeterminate normal and abnormal cells, tissues, and organs. Application of this technique to problems in human pathology and forensic medicine could prove to be very useful.