Disclosed is a cell wall C-polysaccharide antigen of Streptococcus pneumoniae which contains not more than 10% by weight of protein, and preferably less which has been purified with 0.1N Na OH prior to deproteinizing. Also disclosed are polyvalent antibodies raised against Streptococcus pneumoniae which have been affinity purified by passing them over a chromatographic affinity matrix to which is coupled the purified and at least partially deproteinized antigen to render them antigen-specific.