The cause of Forbush decreases is examined by using neutron monitor data and measurements of the interplanetary magnetic field. It is found that for the period examined (December 15, 1965, to April 23, 1966), large enhancements of the interplanetary magnetic field correlate well with decreases in cosmic ray intensity, while various parameters connected with the fluctuations in the field do not display such good correlation. The inference is drawn that Forbush decreases are not related to the turbulence or random motions in the field but to the large-scale features of the field.