The present invention discloses an in-vitro method for transferring biological material into activated NK cells with a pseudotyped retroviral vector particle or a virus-like particle thereof, comprising the steps a) activation of NK cells, and b) addition of said pseudotyped retroviral vector particle or virus-like particle thereof to said activated NK cells, wherein said pseudotyped retroviral vector particle or virus-like particle thereof comprises a modified baboon endogenous retrovirus (BaEV) envelope glycoprotein that is able of binding to and fusing with a hematopoietic cell membrane, thereby transferring biological material into said activated NK cells. Preferentially, the activating of NK cells is performed by the addition of a IL-1 family cytokine to the NK cells.