Two kinds of solid tumors were acquired and scanned in vivo ultrasonically. The first tumor series consisted of spontaneous mammary tumors (fibroadenomas) in rats. The second tumor series were acquired by culturing a carcinoma cell line (4T1-MMT) in culture media. The scatterer properties of average scatterer diameter and acoustic concentration were estimated from a Gaussian form factor using the backscattered ultrasound measured from both kinds of tumors. Parametric images of the tumors were constructed utilizing the estimated scatterer properties for regions of interest inside the tumors. Differences between the two tumor types were clearly seen from the scatterer property estimates and the parametric images. Differences between the 10-MHz and 20-MHz estimates were due to the utilization of a more optimal ka range with the 20-MHz transducer in the estimator. Light microscopic evaluations of the fibroadenomas showed cellular structures of around 100 μm in size, and carcinomas showed cell nuclei with an average size of approximately 15 μm in diameter.