The analysis of life and service strength of bus frame structures should be made at four structural levels: at the level of the complete structure, at that of the components, at the critical cross sections, and for the stress concentrations. This paper presents the fatigue tests and theoretical approximations completes by the AUTÓKUT in relation to the four levels. The great number of ramified tests during the bus development process must be performed in a strict system by harmonising them to each other. The synthesis of these analyses made at the different structural levels can ensure the required service strength and life of bus frames. It is very important to emphasize that the service life is not only a mechanical category at the level of the complete structure, but it depends also on the economic points of view involved.