Immunoglobulin G (IgG) and IgG subclass assays are indicated in patients with suspected primary immunodeficiency (PID). Commercially available assays for IgG subclass determination are calibrated against various preparations, and so specific reference values are required for each of them. Using Optilite® reagents from The Binding Site Group Ltd., we sought to determine the pediatric IgG and IgG subclass reference intervals with respect to the ERM-DA470k certified reference material. Levels of IgG and IgG subclasses were analyzed in serum samples collected from a large cohort of PID-free children and adolescents. Reference intervals were calculated for previously published age groups (6-12 months, 12-18 months, 18 months-2 years, 2-3 years, 3-4 years, 4-6 years, 6-9 years, 9-12 years and 12-18 years), according to the Clinical and Laboratory Standards Institute's C28-A3c protocol. A total of 456 serum samples were analyzed. The correlation between the total IgG and the sum of the IgG subclasses was good (r2=0.96). No statistically significant gender-specific differences were observed. Our results for the changes over time in IgG and IgG subclass levels are consistent with previous reports. The differences between our lower/upper reference limits and those in the literature are probably due to variations in calibration. Our present results provide a reliable basis for the diagnosis of PIDs in childhood and for the accreditation of laboratories using Optilite® immunoturbidimetric reagents for IgG subclass measurement. Laboratory scientists and clinicians should be aware of the need for manufacturer-specific IgG subclass reference intervals.