In the present work, it was aimed to identify radical species produced by gamma irradiation (3-34 kGy) of solid ampicillin, to determine its spectroscopic, dosimetric, stability and kinetic behavior, and to investigate feasibility of the radiation sterilization feature of ampicillin by using electron spin resonance (ESR) spectroscopy. ESR experiments were performed at low and high temperatures (130-400 K) to examine the characteristic properties of the radical intermediates that are produced in ampicillin by gamma radiation treatment. Unirradiated ampicillin presented no ESR signal but irradiated samples exhibited ESR spectra with four resonance peaks spread over a magnetic field range of 8 mT. The spectral parameters of the central resonance line of the spectrum are g = 2.0044 and Delta H-pp = 0.08 mT. An exponential growth function of the applied dose was found to describe best the experimental dose-response data and it was found that ampicillin did not exhibit the feature of a good dosimetric material as its ESR intensity was relatively weak even for the samples irradiated at high level of doses. G(mean) value of gamma-irradiated ampicillin was found to be 4.6 +/- 0.9 x 10(-9) mol/J, which is very small compared to irradiated alanine solid sample. However, the discrimination of irradiated ampicillin from unirradiated one was possible even similar to 3 months after storage at normal conditions. The simulation calculations indicated that gamma irradiation created two different radical species in solid ampicillin. Decay activation energy of the radical species which is mostly responsible from central intense resonance line is calculated to be 55.6 +/- 3.2 kJ/mol by using the signal intensity decay data derived from annealing studies. It was concluded that ampicillin could be sterilized by gamma radiation and ESR spectroscopy can be used as a potential technique to monitor its radiosterilization process.