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The infectivity and the haemagglutinin activity of A2/Aichi/2/68 influenza virus were unchanged during a 2-year storage in allantoic fluid at -80 °C. Over the temperature range from -20 to + 37 °C, exponential slopes could be drawn by means of the regression analysis, the velocity constants showing very low values. Conversely, at + 56 °C inactivation took place in a two-component fashion, each following first-order kinetics. Haemagglutinin and neuraminidase activities were not impaired by exposure to [60Co]-γ-rays (3 × 106 rad), which completely removed infectious particles. The specific rate constants for inactivation of haemagglutinin and neuraminidase activities of untreated and irradiated virus were overlapping when samples were stored for 2 years in the frozen state (- 80 and - 20 °C), whereas a significantly increasing rate of inactivation was recorded for γ-irradiated samples following storage at temperature above 4 °C. Nevertheless, the energy of activation required for thermal inactivation was very low and the entropy of activation showed negative values for both the untreated and the irradiated virus preparations.