Properdin purified to homogeneity from human serum after adsorption to and elution from zymosan (J. Immunol., 100: 142, 1968) is a glycoprotein containing about 9.8% carbohydrate. Preliminary analyses indicate 3.8% hexose, 1.5% hexosamine, 3.8% sialic acid, and 0.7% fucose. The V̄ calculated from amino acid composition and corrected for carbohydrate content was 0.70. Approach to equilibrium ultracentrifugation then yielded a molecular weight (m.w.) 1 of 184,000 ± 12,000. In 6 M guanidine hydrochloride, properdin eluted from a 4% agarose column as a single protein peak with an estimated m.w. of 45,000, suggesting dissociation into four non-covalently linked subunits of similar or identical m.w. Since the possibility exists that zymosan-purified properdin may be in an altered state, properdin has been isolated by affinity chromatography. A column was charged with the IgG fraction of monospecific rabbit anti-human properdin which had been coupled to CNBr-activated Sepharose 4B (1:50 w/w) in 0.06 M barbital buffer, pH 9.6.