Immunohistochemical staining of routinely processed formalin-fixed, paraffin-embedded archival tissue sections with proliferating cell nuclear antigen (PCNA) monoclonal antibody has facilitated the understanding of the regulation of cell proliferation. However, false-negative staining due to masking or poor preservation of antigen epitopes by fixatives or other unknown conditions has been a major problem in PCNA immunohistochemical studies. Microwave heating (MWH) has been used to retrieve antigen in archival tissue. However, optimal staining conditions have not yet been defined. We therefore investigated the effect of MWH with or without a metal solution (zinc sulfate) on routinely processed paraffin-embedded tissue to determine the optimal conditions for retrieval of PCNA. The results were compared with routine immunohistochemical staining. We found that MWH in 1% zinc sulfate solution successfully retrieved PCNA; the optimal MWH time was 7.5 mins. In contrast, when MWH was performed in water, the specific intensity of PCNA expression was not increased because MWH not only enhanced the intensity of the antigen staining but also increased the background staining. In summary, MWH technique in zinc-sulfate solution allowed successful PCNA retrieval from formalin-fixed archival tissue and thus facilitated accurate immunohistochemical evaluation of cell proliferative activity in cancers.