[en] As a preliminary study to image platelet-activating factor (PAF) receptors in vivo, comparative study of biodistribution between 1-O-hexadecyl-2-O-N,N-dimethylcarbamoyl-sn-glycero-3- phosphocholine[choline-methyl-¹¹C](L-[¹¹C]dimethylcarbamoyl-PAF) and nonspecific PAF analog, 3-O-hexadecyl-2-O-N,N-dimethylcarbamoyl-sn-glycero-1-phosphocholine [choline-methyl-¹¹C] (D-[¹¹C]-dimethylcarbamoyl-PAF) was carried out in both normal and tumor-bearing mice. Higher accumulation of L-[¹¹C]dimethylcarbamoyl-PAF than D-[¹¹C]dimethylcarbamoyl-PAF was observed in normal mice spleen. The co-administration of PAF antagonists dose-dependently reduced the radioactivity level of the L-isomer only in the spleen. In mice bearing Ehrlich tumors and Sarcoma 180, more L- than the D-[¹¹C]-isomer was accumulated in the tumor and spleen. We found that specific accumulation sites for L-[¹¹C]dimethylcarbamoyl-PAF exist in the spleen and tumors than in other tissues. Moreover, the comparison of accumulation between L- and D-[¹¹C]dimethylcarbamoyl-PAF would be a useful procedure for estimation of PAF receptors in vivo