Interaction forces between silica surfaces with adsorbed poly (amidoamine) (PAMAM) dendrimers were measured with the colloidal probe technique based on the atomic force microscope (AFM). At small separations, attractive non-DLVO forces are observed. These forces likely originate from interactions between oppositely charged patches on the surfaces, resulting from the heterogeneous distribution of the adsorbed positively charged dendrimers on the negatively charged surface. At large separation distances, the interaction forces are repulsive and consistent with DLVO theory. These forces can be interpreted quantitatively as originating from the overlap of diffuse layers of positively charged surfaces in terms of the Poisson-Boltzmann theory. From the resulting diffuse layer potentials, one can extract the effective charge of the dendrimers in the adsorbed state. This effective charge is about half of the ion condensation value given by Poisson-Boltzmann theory for a charged spherical macromolecule in solution for the different dendrimer generations investigated. This reduction probably originates from the smaller volume available for the diffuse layer of adsorbed macromolecules, but charge neutralization may also play an essential role.