Alphaviruses are a well-characterized group of positive-strand RNA viruses. The identification of cis-acting elements in their genomes and their replication strategy have made them useful as vectors for the expression of heterologous genes. In infected cells, the nonstructural proteins, required for replication and transcription of the viral genes, are translated from the genomic RNA; the structural proteins, the capsid protein that interacts with the RNA to form the nucleocapsid and the proteins embedded in the lipid envelope, are translated from a subgenomic mRNA and can be replaced by heterologous genes. Such modified genomes are self-replicating (replicons); they can be introduced into the cells by transfection and can also be packaged into extracellular particles with defective helper (DH) RNAs. The particular DH RNA determines how well it is replicated and to what extent it is packaged. One potential complication of this system has been that recombination between the replicon genome and the DH RNA may occur. The studies described here were designed to prevent recombination by expressing the capsid protein from one DH RNA and the virus membrane proteins from a second helper RNA. Recombination to yield a nonsegmented infectious virus genome would then require several independent crossover events. There is a translational enhancer located downstream of the initiating AUG in the RNA of the capsid gene that had to be conserved in the second helper to achieve high-level expression of the viral glycoproteins. For this reason, we modified the capsid protein gene in two ways: the first was to use the capsid protein gene from a different alphavirus, Ross River virus, and the second was to make deletions in that gene to maintain the translational enhancer in the RNA but to eliminate the positively charged region in the protein that should be essential for the specific and nonspecific interactions with RNA. Transfections with replicon RNA and the deleted chimeric DH RNA as the only helper resulted in the high-level production of particles that were almost completely devoid of RNA. The inclusion of a helper expressing an intact Sindbis virus capsid protein gene led to the production of high levels of packaged replicons. Recombinants were not detected even after several undiluted passages.