The usual two-fluid models for disperse two-phase flows feature an average disperse-phase stress in the disperse-phase momentum equation. It is shown that it is possible to derive an expression for this quantity without considering the detailed nature of the constitutive relation of the disperse phase. The result, which is more general than other relations of the same nature, is illustrated with a number of examples. It is also shown that a new formulation of the averaged equations recently proposed by the authors is equivalent to the more usual one.