In order to acquire a fundamental knowledge for the development of better tumor-scanning agents, the in vivo incorporation pattern of three 14C-labeled D-amino acids, alanine, leucine, and tryptophan, into the tumor cells and organs of animals bearing Ehrlich mouse tumor, sarcoma-180, leukemia L-1210, or Yoshida sarcoma was investigated, and compared with that of the corresponding L-forms. The radioactivity of D-amino acids tested was most highly found in tumor cells and pancreas, and the activity in tumor cells was several times higher than that of L-forms. A large portion of the radioactivity of D-forms was found in trichloroacetic acid-soluble fraction of the cells, whereas that of L-forms was mostly in protein fraction, except L-alanine. Although the mechanisms whereby the radioactivity of D-amino acids was concentrated more than that of L-forms in the tumor cells have not yet been clearly elucidated, it was concluded that gamma-emitter-labeled D-amino acids themselves or their derivatives might be useful as tumor-detecting radiopharmaceuticals.