Atacicept is a recombinant fusion protein containing the extracellular ligand-binding portion of the transmembrane activator and CAML interactor (TACI, CD267) receptor and inhibits B lymphocyte stimulator (BLyS, CD257) and a proliferation-inducing ligand (APRIL, CD256), both potent stimulators of B cell maturation, proliferation, and survival. Atacicept pharmacokinetics and pharmacodynamics were assessed in a double-blind, placebo-controlled, phase I study in patients with active, moderate to severe rheumatoid arthritis receiving atacicept either as a single subcutaneous or repeated, every other week dose. Pharmacokinetic profiles were determined by measuring serum concentrations of free atacicept and its complex with BLyS. Nonspecific immunoglobulin (Ig)M, IgG, and IgA; IgM-RF (rheumatoid factor), IgG-RF, and IgA-RF antibody levels; and B cell profiles provided markers of biological activity. Pharmacokinetic, biological activity, and relationships between atacicept dose and Ig antibody response were evaluated. Pharmacokinetic profiles of atacicept were nonlinear, influenced by saturable binding with its ligands, but were consistent and predictable. Atacicept treatment reduced Ig and RF serum concentration. IgM antibody levels were most sensitive to atacicept, followed by IgA and IgG, underlining the biological activity of atacicept in patients with rheumatoid arthritis. These findings can be used to explore dosing regimen design scenarios in future studies.