A new method for mammalian cell transformation is proposed which is based on incorporation of plasmids into interpolyelectrolyte complexes (IPECs) with carbon chain polycations. The method is illustrated by examples of pRSV CAT and p beta-Gal plasmid IPECs with poly(N-ethyl-4-vinylpyridinium bromide) (C2PVP) and poly(N-ethyl-4-vinylpyridinium)-poly(N-cetyl-4-vinylpyridinium+ ++) bromides random copolymer (C16PVP). These IPECs are produced spontaneously due to formation of a cooperative system of interchain electrostatic bonds after mixing DNA and polycation solutions. The interaction of IPEC with normal mouse fibroblasts NIH 3T3, human T-lymphoma "Jurkat", and Mardin Darby canine kidney cells has been studied. The data obtained has revealed that plasmid incorporation into IPECs significantly enhances both DNA adsorption on the plasma membrane and DNA uptake into a cell. The in vitro transformation of NIH 3T3 cells was monitored by a standard cloramphenicol acetyltransferase (CAT) assay (pRSV CAT plasmid) and by detection of beta-galactosidase (beta-Gal) expression using 4-methylumbeliferril beta-D-galactopyranoside as a substrate (p beta-Gal plasmid). In both cases it has been proved that IPEC-incorporated plasmids possess an ability for efficient cell transformation. The transforming activity of IPECs depends on their composition and polycation chemical structure. Under optimal conditions the efficiency of cell transformation with IPECs is several fold higher than that observed during standard calcium phosphate precipitation. The mechanism of the phenomenon observed is discussed.(ABSTRACT TRUNCATED AT 250 WORDS)