We evaluated the force-depth compression characteristics of 8 different CPR manikins during mechanical cardiopulmonary resuscitation by a thumper. The force required to compress the manikin's thorax of 1, 2, 3, 4 and 5 cm was measured. It ranged between 6.3 and 14 kp at a depth of 1 cm, 11.6-30 kp at 2 cm, 17-38 kp at 3 cm, 22.5-54 kp at 4 cm and 28.5-69 kp at 5 cm. The manikins with a spring in the thorax (Ambu Man, Ambu MultiMan, Dräger CPR-Max, Laerdal Resusci Anne) as well as one without (Ambu CPR Pal) showed a rather linear relationship between depth and force required to compress the chest. Ambu Man, set at 'High', Laerdal Resusci Anne and Dräger CPR-Max revealed a slight increase in resistance, whereas 2 manikins without a spring (Laerdal Little Anne, Laerdal Family Trainer) and 1 manikin with a plastic spring-like construction (Actar 911) exhibited less resistance with increasing depth. According to our results, the manikins are not uniform in their compression characteristics; some become nonlinear when 3 cm of compression is exceeded. For correct CPR it is of utmost importance that the CPR trainee learns to compress in a sufficiently strong manner, but simultaneously to avoid an exceedingly high depth of compression irrespective of the thorax resistance. In order to prepare the CPR student for the varying chest resistances of the human body, we recommend to train CPR on manikins with different chest resistances.