Exact analytical solutions are obtained for the segregation process of spherical interstellar grains of constant mass under the combined influence of gravitational and drag forces in a uniformly rotating gas cloud. In all cases the gravitational field of the cloud is taken to be that of a uniform sphere. In the case of some selected models, the time required for a grain to reach the equatorial plane of a rotating cloud or the centre of a non-rotating cloud is of order of I0 yr, but generally much longer. In the last two sections we discuss implications of our results to the formation of stars through condensation of molecular hydrogen in the gravitational field of a core of grain material. Cores having masses of about 1026 g and negligible angular momentum are expected to be formed in a period of serveral times I0 yr. Heating of the cores and of the gas by resisted grains is not significant.