It was found that the percentage of IgG-associated agalactosyl N-linked oligosaccharides (G0) falls during normal human pregnancy and rises to values higher than before conception following delivery (n = 10, 39-55 days after delivery). Serial bleeds from a normal pregnant woman showed a fall in the percentage G0 during gestation and a rapid rise post-partum. A similar study on a pregnant arthritic woman with a pathologically elevated percentage G0 also showed a fall in percentage G0 during pregnancy and a rapid rise post-partum. The changes in IgG glycosylation in the pregnant arthritic woman occurred simultaneously with the pregnancy-induced remission and post-partum recurrence of disease. A further seven pregnant women with rheumatoid arthritis were studied and analysis of their G0 values pre- and post-partum confirmed the result. In a further series of experiments using an animal model of rheumatoid arthritis, DBA/1 mice with collagen-induced arthritis were found to have elevated G0 levels compared with control mice. The percentage G0 was found to fall simultaneously with pregnancy-induced remission to the same value as non-arthritic pregnant mice. Post-partum recurrence of arthritis in these mice was also accompanied by a simultaneous and rapid rise in percentage G0. Pseudopregnancy did not result in a change in the percentage G0, confirming the effect of true pregnancy. Since the proportion of agalactosyl IgG is abnormally high in the serum of patients with rheumatoid arthritis these changes in IgG glycoform levels, or the factors which control them, may be related to the mechanisms underlying remission of arthritis in humans during pregnancy.