Several studies have demonstrated that transfer of oncogenes in cultured cells reproducibly induces transmissible alterations in their ganglioside profile; the transfection of the same oncogene into different cell lines and the different localization of the oncogene product result in a different ganglioside expression. In the present study the modifications of the ganglioside pattern in mammary carcinomas induced in transgenic mice by the activated form of the rat neu oncogene have been investigated. Whereas control mammary tissues contain quite exclusively GM3, all neoplastic samples show a substantial decrease of this ganglioside, an accumulation in variable amount of GM3-derived species (GM1, GD3, GD1a, GD1b, GT and GQ) and the appearance of new, not yet identified, sialic acid containing molecules. Interestingly, three out of 10 tumors analyzed, even if histologically comparable to the others but with a larger dimension, show a significative difference as regard to the GM1, GD3 and GD1a content. Our data suggest that an activated oncogene may induce also in vivo a specific and transmissible alteration in the ganglioside pattern, but this distribution could be susceptible to further modifications during the tumor progression.