The experiments were designed to examine whether direct in vivo transfer of plasmid DNA not carrying any 'therapeutic' genes ('empty' plasmid DNA) can influence tumour growth. Murine MC-induced sarcoma Mc12 was transplanted i.m. in syngeneic recipients and the tumour-inoculated mice were treated either with repeated peritumoral (i.m.) injections of the naked plasmid DNA or with a single peritumoral (i.m.) injection of plasmid DNA incorporated in liposomes. Two plasmid DNA preparations, BCMGNeo and pON1 were utilized. The direct in vivo transfer of both 'empty' plasmid DNA preparations inhibited tumour growth and significantly prolonged survival of tumour-bearing mice. Our results emphasize the importance of plasmid DNA controls for evaluation of gene therapy of cancer based on the transfer of 'therapeutic' genes in tumour-bearing individuals.