Activation of c-myc oncogene has been reported in increasing numbers of human ovarian carcinomas and appears to play a role in the biologic behavior of the neoplasms. We have studied the immunohistochemical localization of p-62c-myc, the gene product of c-myc, in 44 cases of serous and mucinous cystadenoma, adenocarcinoma of low malignant potential, and invasive adenocarcinoma, using a monoclonal antibody raised to a synthetic human p62c-myc sequence (Myc 1-6E10). Both serous and mucinous cystadenomas demonstrated a higher frequency of nuclear localization than did carcinomas, which showed much greater cytoplasmic staining, while tumors of low malignant potential showed an intermediate pattern. However, the observed differences did not reach statistical significance. No significant correlation was observed between intracellular localization patterns of p62c-myc and histologic and nuclear grades and mitotic activity in the cases of carcinoma. Great care should be taken in the interpretation of immunohistochemical analysis of oncogene products, especially when attempting to correlate the findings with biologic tumor behavior.