The Zn levels in selected tissue types were measured in a study of the BW7756 murine hepatoma utilizing 65Zn as a tracer. The total body burden and individual tissue activities (including tumor) were recorded at predetermined stages of tumor growth and directly compared to those obtained from normal (healthy) animals. 65Zn fluctuations observed with disease progression were correlated to the growth curve of the carcinoma. Normalization of individual tissue activities to the total body activity, the control of geometry and lack of significant attenuation of the accompanying gamma ray are unique advantages in the use of this isotope.