Abstract Purpose To evaluate the ability of monoclonal antibodies against alkaline phosphatase (AL-P) for tumor immunolocalization in nude mice. Materials and Methods Two monoclonal antibodies against placental alkaline phosphatase (PLAP) and liver alkaline phosphatase (LAP) were intravenously administered to nude mice bearing AL-P producing HeLa Hep 2 tumors. Sera and tumors from 65 patients with testicular germ cell tumors were examined for PLAP levels. Results Levels of PLAP or LAP in the HeLa xenografts were shown to be comparable to levels of PLAP in seminomas, including lymph node metastases, or nonseminomas, respectively. A biodistribution study showed that the percentage of the injected dose of I-125 labelled anti-PLAP in the xenografts was rather constant at around 7 percent until 7 days after injection, while the percentage of the injected dose of I-125 labelled anti-LAP MAb decreased with time as in other tissues which do not contain significant amounts of PLAP or LAP. On scintiscan, the xenografts in nude mice were distinctly visualized at 7 days after injection of anti-PLAP MAb. Conclusions These data may support the potential use of the anti-PLAP MAb for the immunodetection of lymph node metastases of seminoma.