We have purified 24p3 protein from mouse uterine fluid (Biochem. J.316, 545–550, 1996). It is a 25.8-kDa glycoprotein with a N-blocked terminus. This work demonstrated the N-blocked residue to be pyroglutamate, supporting the post-translational cleavage site at Ala-Gln in the precursor protein to generate a putative protein of 180 amino acid residues. Consequently, the two cysteines, Cys78 and Cys177, and the two tryptophans, Trp31 and Trp81, are assigned along the polypeptide chain. No free thiol group was detected in the protein. The presence of formyl-Met-Leu-Phe in the protein solution causes a considerable decrease in the protein fluorescence due to Trp31 and Trp81. Analysis of the fluorescence data supports the idea that the protein can be complexed with the formyl peptide. The association constant for the complex formation is (4.8 ± 0.29) × 105 M-1 at pH 7.4.