Antibodies having light (L) chains encoded by the II-A2 variable region gene segment predominate in the human response to the Haemophilus influenzae type b polysaccharide (Hib PS). To determine whether the closely related homologue of the A2 gene, the II-A18 gene, has the potential to contribute to the repertoire, we examined Hib PS binding to a series of recombinant Fab fragments having either A2 or A18 L chains isolated from a Hib PS-vaccinated adult. The ability to bind Hib PS resided exclusively with those Fab fragments having A2 and containing an insertional arginine at the variable-joining junction. Thus, despite the sequence similarity between A2 and A18, only A2 contributes to the canonical Hib PS paratope.