Purpose: The cross-protection of 7-valent pneumococcal conjugate vaccine ( PCV7) against vaccine-related serotypes has been controversial. We investigated the serological properties of cross-protective antibodies against vaccine-related serotypes 6A, 6C, and 19A induced in young children aged 12-23 months after booster immunization of PCV7 and healthy young adults immunized with PCV7. Methods: IgG and IgM antibody concentrations and opsonic index (OI) against vaccine serotypes 6B and 19F and vaccine-related serotypes 6A, 6C, and 19A were measured by ELISA and opsonophagocytic killing assay (OPKA) in 23 selected immune sera (18 of young children, 5 of healthy young adults). The serological properties and antigenic specificity of protective antibodies were determined by IgM depletion of immune sera, OPKA, and competitive OPKA against serogroup 6 and 19 pneumococci. Results: Immunization of PCV7 to children aged 12-23 months and healthy young adults elicits cross-protection against vaccine-related serotypes 6A, 6C, and 19A. IgM isotype Ab contribute to opsonization to 6A, 6C, and 19A as much as or greater than IgG isotype Ab in results of children. However, IgG is more dominant than IgM in adults immunized with PCV7. Competition OPKA showed that 6B and 19F polysaccharide antigens in PCV7 induce cross-protective antibodies in the form of both IgM and IgG isotypes. Conclusions: Immunization of PCV7 induced cross-protective antibodies against both vaccine serotypes (6B and 19F) and vaccine-related serotypes (6A, 6C, and 19A) in infants and adults. IgM-isotype antibodies induced by PCV7 in infants are more functional in opsonophagocytosis against both vaccine and vaccine-related serotypes than IgM antibodies in adults. OPKA method is better than WHO ELISA for evaluating the functional activity of anti-pneumococcal antibody.