Summary: We study the flow of a multiphase mixture consisting of a viscous fluid and solid particles between two vertical plates. The theory of interacting continua or mixture theory is used. Constitutive relations for stress tensor of granular materials and interaction force are presented and discussed. As application, we study an ideal flow where we assume the flow to be steady and fully developed; the mixture is flowing between two long vertical plates. The nonlinear boundary value problem is solved numerically, and the dimensionless velocity profiles and volume fraction are given as functions of various dimensionless numbers.