Analysis of the heterogeneity of glutathione S-transferase (GST) isozyme expression was carried out by immunohistochemical evaluation of human colon biopsy tissue from 30 patients. Using polyclonal antibodies specific for the GST alpha, mu and pi families of isozymes, an increased expression of pi was found in 21/30 carcinoma specimens compared to their pair-matched controls. This isozyme was the most prevalent in all colon samples. GST mu was expressed at reduced levels in 20/30 carcinoma specimens when compared to normal. GST alpha showed no consistent change. Analysis of the immunostaining in different cell types showed that the highest intensity stain for all isozymes was in the columnar epithelial cells. These cells were primarily responsible for the proportional changes in GST pi and mu between carcinoma and normal tissues. In addition, goblet (crypt), endothelial and muscle cells stained positively. In the lamina propria, lymphocytes and phagocytes stained positively, while fibroblasts, plasma cells and leukocytes were negative. Endocrine cells were also negative. The differential expression of GST pi and mu, confirming biochemical data, supports the potential utility of GST pi as a carcinoma marker.