A direct comparison of IgG1 versus IgG2a anti-CD20 pretreatment in their effect on inhibitor formation. Groups of FVIII-primed FoxP3-GFP/FVIII^−/− mice (n = 9-11) received 250 μg of IgG1 anti-CD20 (A) or IgG2a anti-CD20 (B). Two weeks later, the mice were given an intensive FVIII treatment, which was twice daily intravenous injection of 2 μg FVIII for 5 days. The mice were bled one week after priming with 3 weekly intravenous injection of 0.2 μg FVIII (day 21) and 5 days after the final high-dose FVIII exposure (day 46). Inhibitor titers were evaluated by Bethesda assay. n.s. indicates not significant between IgG1 and IgG2a anti-CD20 pretreatment (Mann-Whitney U test, 1-tailed).