Immunoblot analysis of the 125-kD component isolated by electroelution from preparative SDS-PAGE. A total of 100, 50, and 25 ng purified PR3 from mature neutrophils was applied to a nitrocellulose membrane in dots A, B, and C, respectively; E shows the reaction of the 125-kD component from one SDS-PAGE gel, and D the equivalent volume of electroelution buffer (negative control).