In the case of human serum, factors of the alternate pathway (C3-proactivator (C3PA), properdin, hydrazine-sensitive factor, C3PA-ase) have been characterized or functionally defined and others have been postulated, but only C3PA has been isolated from guinea pig serum. Starting from guinea pig serum without zymosan adsorption properdin was purified by five chromatographic steps, preparative polyacrylamide-gel electrophoresis (PAA) and ultracentrifugation. The main problem was the removal of contaminating γ2-globulins, which was achieved only by the last two isolation procedures. The purified properdin was excluded on a Sephadex G-200 column, had a s-rate of 5.2S, an isoelectric point of pH 8.6, and an electrophoretic mobility of a γ1-protein. In dose-response experiments the properdin-dependence of the C3-turnover was demonstrated for the following inducing substances: zymosan, inulin, γ1-immune aggregates, endotoxic lipopolysaccharides (LPS-S, LPS-R, Lipid-A), pneumococcal polysaccharides S III and the newly tested dextran sulfate. Cobra venom factor (CVF) acted independently of properdin.