[en] The purification of lactoferrin (LF) is reported and its radioimmunoassay applied in the diagnosis of lung cancer (LF-RIA) is established by means of a modified method of extraction. LF (relative molecular weight 75000 Dalton) is obtained. The LF present a single band on both PAGE and double immunodiffusion. The yield of LF is 0.542 mg in 1 mL human milk for this method. The coefficients of variation (CV) within a batch and among batches are 4.77% and 13.59% respectively, and the recovery rate amount to 97.99%. The LF in bronchial washing fluid (BLF) is measured in 36 lung cancer cases, 21 pulmonary infection cases and 4 normal controls. The results suggest that the BLF content in the lung cancer group is higher than that in the pulmonary infection group and the normal control group, and the detection rate is the highest in the lung adenocarcinoma. Pathological analysis indicates that BLF is negatively correlated to the early or later lung tumor