Mice transgenic for the activated rat neu oncogene under the control of the mouse mammary tumor virus long terminal repeat (MMTV-LTR) (neu+ mice), develop breast tumors in 100% of cases. We have previously reported that double transgenic mice obtained from crossing neu+ mice with mice transgenic for the herpes simplex virus thymidine kinase (HSVtk) gene can be used as a suitable model to test the 'suicide gene' strategy for mammary tumor gene therapy in vivo. In the present study, we evaluated the efficacy of the HSVtk/ganciclovir (GCV) system in the neu+ mice by inoculating cells producing a retroviral vector bearing the HSVtk gene in the mammary tumors on one side of the animals, and comparing their weight with that of the contralateral tumors, after systemic GCV administration. A statistically significant effect of this therapy was clearly seen (P < 0.001) but complete eradication of the tumors could not be achieved. This was not due to the inefficient delivery of GCV, as no HSVtk expression was detected in the residual tumors, but could be related to the low transduction efficiency (< 10%) and to inability of the 'bystander effect' (probably due to the absence of functional gap-junctions among mammary tumor cells) to kill nontransduced neoplastic cells. These data suggest that results obtained by in vivo models using transplanted tumor cell lines as targets for gene therapy might not be immediately transferable to spontaneously arising tumors in animals or humans.