Protein 1 (P1)/Clara cell 10 kDa protein is a dimer of identical subunits of 70 amino acids. Designed to expand its clinical significance, a sensitive sandwich enzyme-linked immunosorbent assay (ELISA) for P1 was developed using two monoclonal antibodies, which were recently generated and characterized in this laboratory. The precision, reproducibility and specificity of the assay were quite satisfactory. Species specificity was also satisfactory. The detection sensitivity was 5 ng/l; high enough to quantify P1 in several body fluids. P1 was distributed in the value of 13 to 25 micrograms/l, 153-4300 micrograms/l and 145-8600 micrograms/l, in serum, bronchoalveolar lavage fluids and seminal fluids, respectively.