Coprecipitation of SLPI binding proteins from monocytes using anti-SLPI antibodies. Intact monocytes were surfaced-labeled with Na¹²⁵I. Radioiodinated cells were lysed in 0.5% NP40, and lysate was added to immobilized anti-SLPI antibodies in the presence or absence of SLPI. Proteins binding to the antibodies were eluted using 50 mmol/L glycine, pH 2.5, and 0.15% NP40. Eluted proteins were assayed by PAGE on a 12% gel under nonreducing conditions. The gel was dried and an autoradiogram was made. A major radioiodinated band coprecipitating with SLPI migrated at 55 ± kD (arrow), whereas the same protein did not bind to the antibodies in the absence of SLPI.