b＞Background/b＞ The pneumococcal polysaccharide vaccine ( PPSV) was introduced into the national immunization program for elderly (≥ 65 years of age) in Korea in 2013. To evaluate the immune response in this population, anti-pneumococcal opsonic titers were studied and compared with young adults. b＞Method /b＞ Sera from 90 pneumococcal vaccine-naïve adults were obtained. They were divided into two groups according to their age (25 ~ 51 and 68 ~ 85). The qualitative antibody responses were determined by multiplexed opsonophagocytic killing assay (MOPA) for 12 serotypes (1, 3, 4, 5, 6B, 7F, 9V, 14, 18C, 19A, 19F and 23F) in PPSV before and one month after vaccination. b＞Results/b＞ The geometric mean titers (GMT) for pre and post immunization opsonic titers are shown in Figure 1 and Table 1. Reverse cumulative distribution curves are shown in Figure 2. The GMT increased significantly after immunization for all serotypes in both age groups. However, significantly lower GMTs were observed in the elderly compared to young adults for ten serotypes (1, 3, 4, 5, 6B, 7F, 9V, 14, 18C and 19F). The proportions of four fold increase of opsonic titer were significantly lower in the elderly compared to young adults for seven serotypes (1, 6B, 7F, 9V, 14, 19A and 19F). b＞Conclusion/b＞ PPSV induces a good functional immune response to 12 vaccine-serotypes in both age groups. However, compared to young adults, a significantly lower functional immune response was observed in the elderly.