OBJECTIVE To investigate factors that influence the imaging of placental alkaline phosphatase (PLAP) producing xenografts using an anti-PLAP monoclonal antibody (MAb). METHODS Three xenografts (human seminoma, HeLa Hep 2 cells, and KK-47 bladder cancer cells) each expressing PLAP to a different degree were used to immunolocalize an anti-PLAP MAb, HPMS-1. RESULTS Although the highest PLAP level was found in seminoma xenografts, the MAb was not useful for the imaging of seminoma xenografts because of poor accumulation. Fragmentation of the MAb, such as F(ab)2, however, was shown to be efficient for imaging seminoma xenografts. A distribution study with T1-201 revealed the highest blood flow in HeLa cells and the lowest in seminoma. A difference in blood flow may partially explain the disparity between the amount of MAb accumulation and the level of antigen expression in these three xenografts. CONCLUSIONS Blood flow in targeted tumors was shown to be more critical than their level of antigen expression for the imaging of xenografts with anti-tumor antibody. In addition, fragmentation of the MAb enabled tumor imaging because of a rapid clearance of the fragment from the circulation.