A method for preparing a population of infective, propagation-defective Venezuelan equine encephalitis virus replicon particles (VRPs) collectively encoding and expressing an expression library of a pathogen, parasite or tumor cell, in which the propagation-defective particles cannot produce progeny particles due to the absence from the replicon RNA of at least one sequence coding for a VEE virus structural protein required for packaging the replicon nucleic acid, said method comprising the steps of: a) introducing a plurality of VEE replicon nucleic acids into a plurality of cells, wherein said cells are permissive for VEE replication and packaging, wherein said VEE replicon nucleic acid comprises at least a VEE virus packaging signal and at least one heterologous coding sequence expressible from said VEE replicon nucleic acid, wherein the sequences encoding the structural proteins required for packaging the replicon nucleic acid are distributed among one or more helper nucleic acids and the replicon RNA, and wherein the one or more helper nucleic acids is selected from at least two helper RNA molecules or a DNA molecule, provided that at least one structural protein coding sequence is deleted from the replicon RNA such that the resulting VEE particle is propagation-defective, and wherein the plurality of VEE replicon nucleic acids encode the expression library of a pathogen, parasite or tumor cell, to produce a plurality of modified cells; b) culturing said plurality of modified cells of step (a) under conditions allowing expression of the one or more helper nucleic acids, allowing replication of said VEE replicon nucleic acid and packaging of said VEE replicon nucleic acid to form VRPs; c) contacting the modified cells after step (b) with an aqueous medium containing from 0.2M to 5M of a salt to release the VRPs into the aqueous medium to produce a VRP-containing solution; and d) collecting VRPs from the VRP-containing solution of step (c); wherein the replicon nucleic acid is introduced into said host cell by electroporation; and wherein the cell density in the electroporation milieu is from 10 7 to 5 x 10 8 per mL. a) introducing a plurality of VEE replicon nucleic acids into a plurality of cells, wherein said cells are permissive for VEE replication and packaging, wherein said VEE replicon nucleic acid comprises at least a VEE virus packaging signal and at least one heterologous coding sequence expressible from said VEE replicon nucleic acid, wherein the sequences encoding the structural proteins required for packaging the replicon nucleic acid are distributed among one or more helper nucleic acids and the replicon RNA, and wherein the one or more helper nucleic acids is selected from at least two helper RNA molecules or a DNA molecule, provided that at least one structural protein coding sequence is deleted from the replicon RNA such that the resulting VEE particle is propagation-defective, and wherein the plurality of VEE replicon nucleic acids encode the expression library of a pathogen, parasite or tumor cell, to produce a plurality of modified cells; b) culturing said plurality of modified cells of step (a) under conditions allowing expression of the one or more helper nucleic acids, allowing replication of said VEE replicon nucleic acid and packaging of said VEE replicon nucleic acid to form VRPs; c) contacting the modified cells after step (b) with an aqueous medium containing from 0.2M to 5M of a salt to release the VRPs into the aqueous medium to produce a VRP-containing solution; and d) collecting VRPs from the VRP-containing solution of step (c);