Multiparametric flow cytometry has permitted the quantitative assessment of tumor cell heterogeneity within a single tumor. This technique was applied to four tumor lines with biological activity similar to the variety observed in human prostate cancer. These tumors were derived from the Dunning R3327 transplantable rat prostatic adenocarcinoma and have permitted the development of an index to assess, quantitate, and discriminate variation in the aggressive nature of these tumors. The index is based upon measurements of the mean and the variance for perpendicular light scatter and forward light scatter. Perpendicular light scatter has been shown to correlate with nuclear shape, volume, and roundness as measured by digitized image analysis. Forward light scatter increases in quantity and range as the tumors become more undifferentiated. This technique is rapid, reproducible, and applicable to the quantitative measurement of cytometric parameters on large numbers of individual tumor cells.