The opsonophagocytic-killing assay (OPKA) is one of the primary surrogate assays for evaluating the pneumococcal capsular polysaccharide-protein conjugates under development as vaccines. Because each vaccine contains seven or more different conjugates, multiple OPKA must be performed on each serum. Moreover, the large number of assays can deplete serum samples from infants. To reduce the amount of serum and effort required to conduct OPKA we developed a multi-specificity OPKA using antibiotic resistant pneumococci. Equal numbers of optochin-resistant serotype 6B and streptomycin-resistant 19F pneumococci were used as the target bacteria. Surviving bacteria of each serotype were enumerated by plating on agar containing the appropriate antibiotic. In an examination of 25 immune sera the results obtained with this new assay correlated well with those obtained when bacterial targets were examined individually. By using additional antibiotic resistance markers, more than two specificities can be examined in a single assay. Copyright (C) 2000 Elsevier Science Ltd.