In vitro reactivation of phosphorylated human blood cholinesterases by sodium fluoride was studied. The initial rate of reactivation of Sarin inhibited enzyme was found to be proportional to the concentration of sodium fluoride. The reactivation of Sarin inhibited blood cholinesterases increases with temperature and also when the pH is lowered. Reactivation of Sarin inhibited purified human serum butyrylcholinesterase occurs in two steps with different rates of reactivation. Sarin inhibited crude plasma butyrylcholinesterase is reactivated faster than enzyme inhibited by the thiocholine analogue of Sarin, methyl-isopropoxy-phosphoryl thiocholine iodide. Purified human serum butyrylcholinesterase and human erythrocyte acetylcholinesterase did not show this difference in the rate of reactivation. On a molar basis at pH 7.4 and 37° sodium fluoride was found to be less effective than P2S as a reactivator of Sarin inhibited blood cholinesterases and less effective than Toxogonin or TMB-4 as a reactivator of Tabun inhibited blood cholinesterases. Soman inhibited enzyme could not be reactivated. The mechanism of the reactivation is discussed.