Because IgG1 allotypes might have different half-lives, their influence on infliximab (G1m17, 1 allotype) pharmacokinetics was investigated in a group of spondyloarthritis patients. Infliximab was found to have a shorter half-life in patients homozygous for the G1m17, 1 allotypes than in those carrying the G1m3 with no G1m1 (G1m3,-1) allotype. Because the neonatal FcR (FcRn) is involved in the pharmacokinetics of mAbs, the interaction of different IgG1 allotypes with FcRn was examined using cellular assays and surface plasmon resonance. G1m17, 1 mAbs, such as infliximab and rituximab, were shown to bind more efficiently to FcRn and to be transcytosed better than the G1m3,-1 mAb cetuximab, which explains why infliximab is a better competitor for endogenous IgG1 in G1m3,-1 allotype–bearing patients. A set of four allotype variants of adalimumab (G1m17, 1; G1m17,-1; G1m3, 1; and G1m3,-1) was also tested for its binding to FcRn, revealing that the G1m3, 1 variant, not present in commercial mAbs, binds more efficiently to FcRn and is transcytosed better than the other three variants, all of which are found in therapeutic mAbs.