Residues considered essential for ATP binding to the human P2X(7) receptor (hP2X(7)R) were investigated. HEK293 cells or Xenopus oocytes were transfected with wild-type or site-directed mutants of hP2X(7)R constructs and channel/pore activity measured in the presence of ATP or 2',3'-O-(4-benzoylbenzoyl)-ATP (BzATP). Barium uptake and ethidium influx into HEK293 cells were abolished in cells expressing K193A and K311A mutants, and were partially reduced in cells expressing mutant P210A. K193A and K311A mutations also completely abolished responses to ATP and BzATP in Xenopus oocytes as measured by electrophysiology. These results indicate that K193 and K311 are essential residues in ATP binding in the hP2X(7)R.