Differential scanning microcalorimetry (DSC) has been used to determine the influence of magnesium glutamate on the stability of penicillin G in aqueous solution. The degradation of penicillin is accompanied by an evolution of heat and has been observed as an irreversible, scan rate dependent, broad exothermic transition. The increase of the transition temperature Tm and enthalpy change DeltaH with increasing magnesium glutamate concentration indicates the increase of penicillin G stability. The kinetic parameters describing the penicillin decomposition process, obtained for a reaction following a first-order course, suggest maximum penicillin G stability if about two molecules of salt per one penicillin molecule are present in solution.