The cellular localization of alpha-1-antichymotrypsin (alpha 1-ACT) was studied immunohistochemically using rabbit HRP-labeled Fab' against human alpha 1-ACT. alpha 1-ACT was found in cell nuclei of carcinomas of the stomach, liver, breast, pancreas and leiomyosarcoma and in cell nuclei of lymphoid cells infiltrated into the stomach carcinoma mass. alpha 1-ACT was not found in carcinoma cells of the colon, uterus, rectum or esophagus, or in lymphoid cells infiltrated into the rectal carcinoma mass or into inflammatory regions such as gastric ulcers or appendicitis. Further, alpha 1-ACT was not found in normal cells around the carcinoma mass or in normal tissues.