For a set of 11 magnetic cloud events, the time-integrated values of the injection function necessary to observe variations in the Dst index at levels of −50, -75, -100, -125 and −150 nT have been estimated. A study of the average values of magnetic clouds with different polarity has shown that the subset of magnetic clouds with N-S polarity has time-integrated values greater than those for magnetic clouds with S-N polarity. This result agrees with a model calculation.