We propose to solve revenue-maximizing selling mechanisms for a multiproduct monopoly based on nonlinear pricing à la Bulow and Roberts [1989]. We derive the marginal revenue change due to a price perturbation on any subset of bundles, holding the prices of other bundles fixed. We show that the MR of any bundle can be expressed as the integral of an "MR density" over the set of types who self-select this bundle (demand set), minus the mass of types on the boundary of its intersection with the type space. Given any optimal taxation mechanism, the revenue must not increase with any small price change for bundles with positive demand (MR = 0), nor with a small price decrease for bundles with zero demand (MR ≥ 0).