The radioimmunoassay (RIA) of pneumococcal capsular polysaccharide antibodies is dependent on the association of radiolabeled antigen and pneumococcal antibody. However, it is not known whether the ability of the antibody to complex with antigen correlates with in vivo protection against infection. A method for evaluating protective ability of antibody vis-à-vis binding ability is to passively transfer a measured quantity of antibody into recipient mice followed by a lethal challenge with virulent pneumococci. Protection against a fatal outcome is then correlated with the amount of antibody (as measured by RIA) passively transferred. This comparison of quantitation by RIA and biological protection in mice was performed on 30 sera from humans. The sera were obtained from vaccinated healthy persons and vaccinated persons at high risk of developing pneumococcal infection, including people with nephrotic syndrome, chronic obstructive pulmonary disease and various forms of cancer. The results of these studies indicate that antibody as measured by RIA correlates with protective antibody against pneumococcal infection. These studies were conducted on pneumococcal serotype 3.