We have compared the abilities of IgG1, IgG(2b), and IgG3 antibodies to protect mice against infection with a bacterial pathogen for which antibody-mediated protection is thought to be a function of killing by phagocytes rather than complement-mediated lysis. For this study, we used anti-phosphocholine (PC) antibodies of the T15 idiotype, since they are known to have very similar binding sites and have the ability to protect mice from Streptococcus pneumoniae infection. Our results indicated that antibodies with the three different IgG constant regions gave similar degrees of protection. This finding was surprising, since earlier data showed a strong restriction of anti-carbohydrate and anti-PC antibodies to the IgG3 subclass. Our data raise the possibility that the restriction of most immune responses to particular IgG subclasses may be more to allow isotype-specific regulation than to utilize special effector functions of different IgG isotypes.