Biomarkers for lung cancer would greatly improve our ability to detect, monitor, and treat patients with this deadly disease. Proteins secreted by pulmonary epithelial cells are excellent candidates for such biomarkers. We have previously reported1 that murine urethane-induced adenomas expressed surfactant proteins (SPs) including SP-D. To determine whether serum levels of SP-D would be a useful biomarker of parenchymal and airway diseases in mice, we developed an enzyme-linked immunosorbent assay using recombinant murine SP-D as the standard. The mean (± SD) serum levels of SP-D in normal mice were 5.0 ± 0.2 ng/mL (121 mice), in SP-D nulls were 0.04 ± 0.03 ng/mL (5 mice), and in SP-D transgenic mice were 63.6 ± 9.0 ng/mL (3 mice). In mice with urethane-induced adenomas, the mean levels were 50.8 ± 6.4 ng/mL (5 mice), and in mice with tumors regulated by an inducible mutated K-ras the levels were 158 ± 20 ng/mL (12 mice).