The search for tumor-avid agents for use in nuclear medicine imaging is an ongoing field of importance. The purpose of this study was to determine the affinity for radiolabeled vitamin B12 by a wide histologic variety of tumor types in mice. Seventeen different types of tumor were grown subcutaneously in female Balb/C or Balb nu/nu(nude) mice. When the tumors reached about 1 cm in diameter, mice were injected intraperitoneally with 57Co-vitamin B12. Twenty-foul hours later, the mice were sacrificed. Organs and tissues were removed, weighed, and activity per mg determined by gamma counter. Values represented cpm/mg tissue that was normalized to 20 grams body weight for each mouse. A wide variety of tumor types showed significant uptake and concentration of 57Co-vitamin B12, as evidenced by tumor:tissue activity ratios. For many tissues of great importance in terms of background(bone, muscle, blood), the tumor:tissue activity ratios of uptake were high. These data strongly suggest that further efforts to evaluate the utility of radiolabeled adducts of vitamin B12 for clinical use in oncologic imaging are warranted.