An anti-complementary polysaccharide, DWA-2, isolated from an unossified pilose antler of C. nippon Temminck by digestion with pronase, gel filtration, and affinity chromatography, consisted mainly of GalNAc, GlcA, IdoA, and sulfate in the molar ratios 1.0:0.6:0.3:0.8, and small proportions of Man, Gal, GlcNAc, and protein (4.5%). Methylation analysis, NMR spectroscopy, and degradation with enzymes indicated that DWA-2 contained chondroitin sulfate A-, B-, and C-like moieties. DWA-2 showed potent anti-complementary activity, and crossed immunoelectrophoresis indicated that it cleaved complement C3 in the absence of Ca2+ ion. Digestion of DWA-2 with chondroitinase ABC or ACI reduced the anti-complementary activity to a low level, but digestion with chondroitinase B reduced the activity by approximately 40% and the enzyme-resistant fraction still showed a significant activity.