The potency of clinical group B streptococcal (GBS) capsular polysaccharide-protein conjugate vaccines has been assessed with use of a mouse maternal vaccination-neonatal pup challenge model of GBS disease. Two of the three conjugated GBS vaccines bottled as liquid preparations showed a potency reduction from 100% to < or = 50% in 3 years; whereas all six vaccines bottled as lyophilized preparations with sucrose excipient exhibited no loss of potency during the same span of time. A reconstituted GBS conjugate vaccine remained potent and antigenically intact after 31 days' storage at 2-8 degrees C. These data suggest that lyophilization in the presence of sucrose extends the potency of GBS conjugate vaccines.