Abstract: Objective To establish a time resolved fluoroimmunoassay (TRFIA) for detecting the carbohydrate antigen CA50 and evaluating its diagnostic and clinical application in the treatment of malignant tumor. Methods A solid-phase sandwich TRFIA of CA50 was established. The europium (Eu³⁺) was labelled on the monoclonal antibody (McAb) against CA50 and then coated on the microplate as a carrier. The serum contents of CA50 in 76 patients with malignant tumor, 47 patients with carcinoids and 213 normal controls were measured using clinical diagnostic methods to evaluate the clinical application of CA50 TRFIA in the diagnosis of tumor. Results The sensitivity of the CA50 TRFIA was1.08×10³u/L. The intra-and inter assay CV were 2.6%nd 3.5%, respectively. The recovery rate was 108.61%. Its correlation coefficient to IRMA was 0.904. The measurement range was (1.08～140)×10³ u/L. There was no significant difference between the patient group with carcinoids and the normal control group (P>0.05). Compared with the patient group with carcinoids and the normal control group, the patient group with ovarian tumor, alimentary canal carcinoids and lung cancer exhibited statistical significant difference (P<0.01). Conclusion The established sandwich TRFIA for CA50 measurement is an exceedingly specific,accurate and steady assay. It is also radiation free which can be served as an auxiliary index in the diagnosis of tumor.