Three strategies have been designed to concentrate infectious retroviral vectors from the supernatants of human- (HT1080) and murine- (NIH 3T3) based packaging cells. Streptavidin-conjugated paramagnetic particles in conjunction with (i) antibodies directed against murine fibronectin, (ii) biotinylated lectins, or (iii) biotin-modified packaging cell-surface proteins allow affinity-mediated magnetic concentration of retroviral vectors. Retroviral titers (assayed by colony formation of human myeloid K562 cells) are increased by 1-4 x 10(3)-fold after volume reductions of only 125-fold. Using these procedures, preparations of 5 x 10(8) cfu/ml are routinely made from relatively low-titer (2-5 x 10(5) cfu/ml) starting material. High-titer (paramagnetic) retroviral vector preparations can be used for magnetic field-dependent retroviral infection in vitro. Magnetic field-dependent localization such as this may enable the in vivo administration of formulations that concentrate retroviral infection to the required target tissues and organs.