Using a PAP immunoperoxidase technique, we have demonstrated the plasma protein prealbumin in 92 (87 per cent) out of a series of 106 carcinoid tumours from various sites. The frequency of detection was greater than with the conventional stains; 83 (78 per cent) of the cases were positive for one or more of these stains. The Grimelius technique was by far the most effective of the conventional stains; it was positive in 81 (76 per cent) cases. We were unable to detect prealbumin in a variety of carcinomas. Although prealbumin identification did not appear to have any advantage over the routine stains in midgut-derived carcinoids, it was much more effective in the diagnosis of bronchial (foregut) and rectal (hindgut) carcinoids. We believe that prealbumin staining offers a cheap, reliable and valuable method for the detection of carcinoid tumours in routine histopathology.