The α,μ and π classes of glutathlone S-transferase (GST) were evaluated as early immunocytochemical markers for the development of atypical foci within the pancreases of azaserine treated rats. Changes detected with haematoxylin and eosin (H&E) were compared with those detected by immunocyto chemistry using antibodies raised against each class of GST. All foci detected with H&E staining were classified as acidophilic atypical acinar cell nodules (AACN), which have previously been reported in this model. All of these AACN overexpressed GST μ. However, 64% of foci detected with GST staining had not been identified as AACN during a prior examination with H&E. Re-evaluation of the H&E sections revealed that some of these foci showed subtle morpho logical changes which are indicative of AACN. In many cases, however, no morphological difference could be seen with H&E staining. We conclude that immunocytochemical stain ing for GSTμ t is a more reliable and sensitive method than H&E for detecting the early stages of azaserine-mduced foci. Furthermore, we suggest that studies on the incidence and growth of these foci can be shortened considerably if GST μ staining is used in conjunction with H&E.