Fresh tissue of human choriocarcinoma was injected subcutaneously into the BALB/C nude mouse. The xenografts had grown up with 16-68 passages. The hCG level of the ascites or intratumoral blood was 400-20,000 mIU/ml. Radioimmunodetection was performed using 131I labelled murine anti-hCG McAb. The labelled antibody could be specifically accumulated in the tumor sites and the tumor imaging was acquired. The ideal imaging time was 72-96 hours after the injection of antibody.