Background: A group B Streptococcus (GBS) maternal vaccine comprising the most prevalent serotypes is the ideal measure to prevent neonatal GBS disease. Opsonophagocytosis killing assay (OPKA) is a good correlate to estimate vaccine efficacy, but it has not been comprehensively performed with GBS strains from Latin America. Methods: Here, the GASTON OPKA was performed using standard human reference sera (SHRS) among 124 GBS isolates from anovaginal carriage (n=64), semen (n=47), and disease (n=13). Isolates belonged to serotypes Ia, Ib, II, III, IV or V; all included in current vaccine proposals under development. The opsonophagocytic titer was defined as the serum dilution where 50% killing was observed. SHRS was kindly provided by Professor Carol Baker. Results: All GBS isolates were susceptible to killing using SHRS of the matched serotype, but with a large range of titers. Significant lower titers were observed for GBS isolates from semen when compared to anovaginal carriage isolates (geometric mean titers: 5,166 versus 17,697; P<0.0001). Within each clinical source evaluated, a subset of isolates presented low titers (anovaginal n=17, semen n=32, invasive disease n=10) and those were mainly associated with serotypes Ia (n=20), V (n=20), and III (n=8). Conclusion: Our results show a potential good efficacy of current serotype-based vaccines among GBS isolates recovered in our setting. All strains were killed in the GASTON OPKA by serotype-specific serum but a subset of strains showed reduced susceptibility to killing.