The ectodomain of human Fc gamma RI (rsCD64) was expressed in HEK 293T cells and purified by immobilized-metal affinity chromatography. Binding activity to human IgG was verified by ELISA and the isotype-specificity determined by a surface plasmon resonance inhibition assay was found to be the same as for native CD64. The active concentration of the rsCD64 preparation was derived using a solution competition assay and was used for the subsequent kinetic analysis. Binding curves were well described by a simple monovalent interaction model confirming the known stoichiometry of the interaction. Mass-transport limitation was prevented by using sufficiently low surface capacities. For binding to the recombinant mouse/human chimeric antibody cPIPP (IgG1/K) a high association rate of k(ass) = 1.7 x 10(6) (M s)(-1) and a low dissociation rate of k(diss) = 1.8 x 10(-4) s(-1) were observed. The derived dissociation equilibrium constant of K-D = 110 pM was significantly lower than that reported for binding to native Fc gamma RI. (c) 2005 Elsevier Inc. All rights reserved.