IgG was purified by gel filtration of sera from 3 adults immunized with PRP. The ratio of IgGl to IgG2 anti-PRP was determined in affinity purified anti-PRP. We also determined IgG1 anti-PRP by measuring anti-PRP radioantigen binding activity before and after column immunoabsorption of IgGl employing a highly specific monoclonal prepared locally (HG-11). 38–46% of the post-vaccine serum IgG anti-PRP was of the IgGl subclass. An ELISA was then used to measure IgG and IgGl anti-PRP antibody in sera from 16 normal adults; and from 97 children 12 to 83 mos of age and 8 adults immunized with PRP. Only 2 of 16 unimmunized adults had IgGl liters >1:49. The IgGl anti-PRP antibody 2 mo after immunization was age-dependent.12 of 35 children had high IgG antiPRP responses (post vaccine titers >1:400) but no detectable IgGl antibody (IgGl titer <1:49). Thus, IgGl is an important component of the IgG antiPRP response of some but not all immunized subjects. In addition, although serum IgGl reaches adult concentrations in the first year of life, most children of this age cannot respond to immunization with IgGl anti-PRP whereas many older children can.