Gene transfer to provide long-term expression of a therapeutic product, without introducing unwelcome genetic information, is a goal being sought for therapy of both hereditary and acquired diseases. Polyoma virus pseudocapsids, generated from a VP1-expressing recombinant baculovirus, lack viral DNA and have been successfully used to introduce small exogenous genes stably into cells in vitro by a process designated 'pseudofection'; although pseudocapsids protect only about 3 kbp of exogenous DNA, low efficiency transfer of a larger fragment (6.2 kbp) has been observed. Here, expression of a 7.2 kbp plasmid (pCMV beta) encoding the beta-galactosidase gene was assessed to monitor not only efficiency, but the ability of pseudocapsids to transfer larger-sized DNA on their own, or in the presence of the polycation, poly-L-lysine, added to protect nonencapsidated DNA. When complexed to pseudocapsids only, the efficiency of expression of the transferred beta-galactosidase gene (in human or rodent cells), although low, appeared to stabilise with time. In the presence of polylysine, unencapsidated DNA was shown to be protected against DNase activity, but electron microscopy (EM) revealed the formation of large mixed aggregates. The addition of pseudocapsids to these aggregates, and measurement of mobilities of the complexes in CsCl equilibrum centrifugation, indicated that they contained negligible amounts of VP1. For subsequent pseudofection experiments, DNA was complexed first with pseudocapsids, then polylysine was added. The latter did not appear to displace pseudocapsids from DNA, and was found to increase the efficiency of short-term expression both in in vitro and in vivo experiments. Gene expression, analysed histochemically or by the polymerase chain reaction, revealed transcriptional activity of the input gene, with expression first diminishing, then stabilising over time. The presence of pseudocapsids, in complexes with DNA with or without polylysine, allowed for stable and persistent gene expression.