Immunohistochemical staining pattern of proliferating cell nuclear antigen (PCNA) was examined in rat regenarating liver specimens fixed under the various conditions (10% buffered formalin for 6 hr, 1% paraformaldehyde for 6 hr, 100% methanol for 4 hr, 70% ethanol for 6 hr, 100% acetone for 2 hr and Carnoy solution for 4 hr). Most distinct granular staining pattern of PCNA was observed in fixation with 100% methanol or 70% ethanol. Specimens fixed with 10% buffered formalin or 1% paraformaldehyde showed both distinct granular and slight diffuse pattern of PCNA staining. Specimens fixed with 100% acetone showed only slight diffuse staining. Specimens fixed with Carnoy solution showed little staining. Number of PCNA positive cells in the specimens fixed with 100% methanol or 70% ethanol on 24 hours after hepatectomy coincided closely with those of BrdU positive cells, which is known to be a representative marker of cell proliferation. In contrast, dissociative result was obtained in specimens fixed with 10% buffered formalin, 1% paraformaldehyde, 100% acetone or Carnoy solution when numbers of PCNA positive cells were compared with those of BrdU. These findings suggest that tissue specimens fixed with 100% methanol or 70% ethanol is useful for evaluating the extent of cell proliferation.