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Diverse polycationic polymers have been used as nonviral transfection agents. Here we report the ability of colloidal silica particles with covalently attached cationic surface modifications to transfect plasmid DNA in vitro and make an attempt to describe the structure of the resulting transfection complexes. In analogy to the terms lipoplex and polyplex, we propose to describe the nanoparticle−DNA complexes by the term “nanoplex”. Three batches, Si10E, Si100E, and Si26H, sized between 10 and 100 nm and with ζ potentials ranging from +7 to +31 mV at pH 7.4 were evaluated. The galactosidase expression plasmid DNA pCMVβ was immobilized on the particle surface and efficiently transfected Cos-1 cells. The transfection activity was accompanied by very low cytotoxicity, with LD₅₀ values in the milligrams per milliliter range. The most active batch, Si26H, was produced by modification of commercially available …