The interactions between poly(amidoamine) dendrimers with surface carboxylic groups for generations G(0.5-5.5) and positively charged alumina particles were investigated at pH 5 by measuring the adsorbed amount of dendrimers, the zeta potential, and the dispersion stability of alumina suspensions. The amount of dendrimers adsorbed increased with increasing concentration of dendrimers and then attained a saturation, where the saturated amount in weight of dendrimers adsorbed became larger from G(0.5) to G(5.5). In addition, alumina particles were flocculated over a wide concentration region of the earlier generations G(0.5-1.5), but they showed a dispersion-flocculation-redispersion sequence with dendrimer concentration for the later generations G(2.5-5.5). The zeta potentials of alumina particles decreased with increasing concentration of dendrimers as well as increasing generations of dendrimers. These results indicate that the earlier denderimers operate as electrolytes and that the later dendrimers behave like an anionic surfactant or polyelectrolyte for alumina dispersion.