Oncogene-bearing transgenic mice develop various kinds of tumors depending on both the regulatory sequences and the specific oncogene used. These mice not only help to clarify the pathogenetic pathways leading to tumor formation, but can also be useful as models to test novel therapeutic strategies, including gene therapy. We have previously reported the establishment of an MMTV-neu (ErbB-2) transgenic mouse lineage, in which 100% of females develop breast tumors with many features similar to their human counterparts; these tumors are due to the over-expression of the activated rat neu oncogene in the mammary gland. From one such mouse we established a cell line of mammary adenocarcinoma named MG1361. We report here that the growth of this cell line can be inhibited in vitro and in vivo by transfection of a plasmid vector carrying an antisense anti-neu construct. This inhibitory effect is specific, as it is related to the expression of the antisense transgene (determined by RT-PCR), and to a reduction in neu mRNA and protein, as determined by Northern and Western blot analyses. Moreover, inoculation of cells carrying the antisense or the control vector in nude mice demonstrated that the morphological and biochemical effects elicited by the antisense construct resulted in a significantly slower rate of in vivo growth of tumor xenografts. Finally, significant mammary tumor growth inhibition was obtained after liposome-mediated direct inoculation of the same antisense vector in tumors spontaneously arising in MMTV-neu mice. Taken together, these findings suggest that targeting neu expression by an integrated large anti-neu antisense segment affects the in vivo growth of these tumors.