It has been proposed that cell-free nucleic acids in the plasma participate in tumorigenesis and the development of metastases via transfection-like uptake of such nucleic acids by susceptible cells. This putative phenomenon is tentatively referred to as "genometastasis." In the present study, we examined the effects on cultured cells of plasma from healthy individuals and from patients with colon cancer. Cultures of NIH-3T3 cells and human adipose-derived stem cells (hASC) were supplemented with samples of plasma from patients with K-ras-mutated colorectal tumors or from healthy subjects using two different protocols: direct addition of plasma to cultures in standard plates and addition in the absence of contact between plasma and cells, which were separated by a membrane with 0.4-mum pores. In plasma-treated hASCs, no K-ras-mutated sequences were detected by real-time PCR. In contrast, in most cultures of plasma-treated NIH-3T3 cells (murine cells), the transfer of human DNA occurred, as verified by the detection of human K-ras sequences, p53 sequences, and beta-globin-encoding sequences. Moreover, NIH-3T3 cells that had been cultured with plasma from patients with colon cancer were oncogenically transformed, as shown by the development of carcinomas in nonobese diabetic-severe combined immunodeficient mice after the injection of such cells. Microscopic analysis of membranes that had separated plasma from cultured cells confirmed the complete absence of cells in the plasma. We only observed noncell particles, having diameters of <0.4 mum. Our results indicate that plasma from cancer patients is able to transform cultured cells oncogenically, supporting the previously proposed hypothesis of genometastasis.