A new theory to study isotropic-nematic transition is proposed. This theory requires a good knowledge of the thermodynamic properties of the isotropic phase. It allows to study nematic formation in systems possessing attractive forces. We determine isotropic-nematic equilibria fora number of hard linear models as hard spherocylinders, hard ellipsoids, and hard tangent spheres in a linear configuration. The theory predicts quite nicely the transitions when compared to simulation results. We also study the effect of an ideal dipole or quadrupole on nematic formation. Dipolar or quadrupolar forces favor the presence of a nematic phase although the effect is moderate. However, for large multipole moments no stable nematic phase was found.