In order to develop a safe and effective systemically-administered delivery system for solid tumors, the biodistribution of control gelatin and poly(ethylene-glycol) modified (PEGylated) gelatin nanoparticles was examined in Lewis lung carcinoma (LLC)-bearing female C57BL6 mice. Type B gelatin and PEGylated gelatin nanoparticles were radiolabeled (125I) for the in vivo biodistribution studies after intravenous (i.v.) administration through the tail vein in LLC-bearing mice. At various time intervals, the tumor-bearing mice were sacrificed and tumor, blood, and major organs were harvested for analysis of radioactivity corresponding to the localization of the nanoparticles. Percent recovered dose was determined and normalized to the weight of the tissue or fluid sample. Non-compartmental pharmacokinetic analysis was performed to determine the long-circulating property and preferential tumor targeting potential of PEGylated gelatin nanoparticles in vivo. From the radioactivity in plasma and various organs collected, it was evident that the majority of PEGylated nanoparticles were present either in the blood pool or taken up by the tumor mass and liver. For instance, after 3 hours, the PEGylated gelatin nanoparticles were almost 2-fold higher in the blood pool than the control gelatin nanoparticles. PEGylated gelatin nanoparticles remained in the blood pool for a longer period of time due to the steric repulsion effect of the PEG chains as compared to the control gelatin nanoparticles. In addition, approximately 4-5% of the recovered dose of PEGylated gelatin nanoparticles was present in the tumor mass for up to 12 hours. The plasma and the tumor half-lives, area-under-the-curve, and the mean residence time of the PEGylated gelatin nanoparticles were significantly greater than those of the control gelatin nanoparticles. The results of the study confirmed long-circulating property and preferential tumor targeting potential of PEGylated gelatin nanoparticles in a murine tumor model.