Using an original microcalorimetric method, the existence of the Mg2ATP ternary chelate has been studied. The thermodynamic parameters of this complex are delta H = 7.2 +/- 0.5 kJ mole-1 and K = 49 +/- 9 M-1. These values are compared with those previously obtained for binary chelate Mg ATP2-. A possible regulation role of Mg2ATP is discussed.