The galactosylation status of IgG from both control and arthritic DBA‐1 mice was determined by exoglycosidase sequencing and an anti‐GlcNAc monoclonal based ELISA. Two to three weeks after arthritis onset mice with collagen‐induced arthritis (CIA) showed a modest increase in IgG anti‐GlcNAc reactivity against controls (0.644 ± 0.080 and 0.530 ± 0.087, respectively, mean absorbance ± SEM n = 4) consistent with previous literature reports. However, the authors were unable to detect any significant changes in the galactosylation of purified IgG from arthritic and control mice using direct sequencing techniques (percentage G0 of 34.5% ± 1.9 and 37.7% ± 2.4, respectively, mean ± SEM n = 4). It has been demonstrated that a correlation exists between percentage G0 and anti‐GlcNAc reactivity for purified human IgG and sera samples, respectively; no such correlation was found for murine IgG and sera. The galactosylation of purified polyclonal murine IgG isotypes was analysed on a separate group of arthritic mice selected for their high anti‐GlcNAc reactivities. No significant differences were observed between control and arthritic mice. Immunoglobulin G isotype specific differences were found, with IgG₁ exhibiting the highest percentage G0 (45–48%) followed by IgG_2a (27–37%), IgG₃ (20–32%) and IgG_2b the lowest (13–17%). The percentage G0 and anti‐GlcNAc reactivity of purified IgG₁ and IgG_2b showed a narrow range of values when compared to those of IgG_2a and IgG₃ samples. Pooled sera from both arthritic and control mice was used to purify large quantities of IgG₃, which on analysis revealed a fourfold increase in anti‐GlcNAc reactivity in the arthritic sample compared to control. Paradoxically these same IgG₃ samples contained similar percentage G0 levels as determined by direct sequencing. The results suggest that IgG₁ and IgG_2b exhibit isotype selective oligosaccharide processing as little sample heterogeneity could be observed. These two purified IgG isotypes displayed a good correlation between percentage G0 and anti‐GlcNAc reactivity; this was in contrast to IgG_2a and IgG₃. Immunoglobulin G₃ from arthritic mice may have G0 oligosaccharides selectively paired together with no net increase in percentage G0. This observation is discussed in detail as is the role of agalactosyl IgG in murine type II collagen‐induced arthritis.