The correlation between perturbations of the interplanetary medium and Forbush decrease events is studied by using interplanetary-magnetic-field and cosmic-ray data for the period March to November 1968, with the purpose of investigating the particular mechanism responsible for this type of modulation of the cosmic-ray intensity. The main results can be summarized as follows: 1) The major and more unequivocal Forbush-like events are associated with interplanetary discontinuities of the type of hydromagnetic shock waves. 2) The decreasing stage of a Forbush decrease takes place entirely while the perturbed region sweeps over the Earth. 3) Irregularities of a scale size comparable to the gyroradius of approximately 10 GeV/c rigidity cosmic-ray particles are enhanced in the perturbed regions. And the Forbush-decrease amplitude seems to be related to the amount of field fluctuations.