Image analysis of pathologic specimens with emphasis on quantitation of cell proliferation has been underway over the past two years. The focus has been on utilizing state-of-the-art computer software to quantitate cell turnover in liver and lung tissues following exposure to genotoxic and nongenotoxic carcinogens. Development of immunohistochemical procedures for demonstrating S-phase nuclei following administration of bromodeoxyuridine offers great promise as a useful technique. In addition, new methods for demonstrating proliferating cell nuclear antigen have recently been established in our laboratory. The new techniques are being applied to tissues of mice exposed to methylene chloride by inhalation for varying periods of time, including a two-year carcinogenesis study. Standard morphometric techniques are being used to chart development and growth of preneoplastic lesions and tumor progression. This work will continue for the next two years.