The biologic activity of different human IgG subclass antibodies directed against the Haemophilus influenzae type b (Hib) capsular polysaccharide (PRP) was compared by using an in vitro complement-mediated bactericidal assay and an in vivo passive protection assay in infant rats. An IgG pool was made by Sephacryl S-300 chromatography of sera from adults immunized with PRP vaccine. An IgG2 subclass fraction was prepared by column immunoabsorption of the IgG pool with anti-IgG1 monoclonal antibody. An IgG1 subclass fraction was eluted from the affinity matrix. IgG1, IgG2, IgG3, and IgG4 concentrations in the fractions were measured by solid-phase competitive radioimmunoassays, and anti-PRP antibody was measured by a modified Farr assay. Each fraction was >90% pure IgG2 or IgG1, respectively. There were no significant differences in the minimal anti-PRP antibody concentrations required to kill 50% of Hib cells in vitro (IgG, 0.22; IgG1, 0.21; and IgG2, 0.42 μg/ml). Similarly, equivalent amounts of anti-PRP antibody of the IgG1 or IgG2 fractions protected against bacteremia (IgG1, 0.12; IgG2, 0.24 μg per rat). IgG absorbed to remove anti-PRP antibody was neither bactericidal nor protective. Thus IgG1 and IgG2 anti-PRP antibody have equivalent functional activities against Hib as determined by these biologic assays.