In the microscopic examination of gastric cancer, it is sometimes difficult to identify scirrhous carcinoma cells, because they grow individually into the fibrous tissue. In this study we used an anti-scirrhous carcinoma monoclonal antibody S202, which was reactive with 100 per cent of the cells belonging to this type of carcinoma, in a rapid immunostaining procedure to identify the limit of tumor invasion at the time of surgery. S202 was applied using this new immunostaining method and the results compared with those obtained by the routine immunohistological method. Five minutes incubation was sufficient to stain the tumor cells with biotin-conjugated S202 and the entire procedure took only 14-15 mins. There was no difference between the new and routine methods in terms of distribution of S202-positive carcinoma cells. This rapid staining method enabled a clearer recognition of infiltrating tumor cells in the frozen sections of tissue specimens, than was previously possible. This technique will thus make it possible to determine the limit of tumor extension in resected tissues accurately and rapidly during surgery.