Human natural killer (NK) cells play a key role in anti-cancer and anti-viral immunity, but their selective amplification in vitro is extremely tedious to achieve and remains one of the most challenging problems to solve for efficient NK cell-based immuno-therapeutic treatments against malignant diseases. Here we report that, when added to ex vivo culture of peripheral blood mononuclear cells from healthy volunteers or from cancer patients with multiple myeloma, poly (phosphorhydrazone) dendrimers capped with amino-bis(methylene phosphonate) end groups enable the efficient proliferation of NK cells with anti-cancer cytotoxicity in vivo. We also show that the amplification of the NK population relies on the preliminary activation of monocytes in the framework of a multistep cross-talk between monocytes and NK cells before the proliferation thereof. Thus poly(phosphorhydrazone) dendrimers represent a novel class of extremely promising drugs to develop NK-cell based anti-cancer therapies.