The molecular-dynamics method is used to determine the equation of state and the viscosities of systems of particles interacting through potentials assumed to represent the potential between krypton atoms. Three potentials are considered: two Lennard-Jones potentials (12-6, 9-6) and a potential reproducing the properties of krypton at very low density and in the solid phase. The equation of state and the viscosity are compared with the values measured on krypton in the fluid phase at 50°C. It appears from this comparison that none of the potentials used are compatible with experiment. For the Lennard-Jones potentials, it seems unlikely to obtain any agreement between the calculated and experimental values for dilute and dense gases by varying the values of the parameters defining them. For the third potential such an agreement is obtained by varying the values of the parameters defining it.