ix-week-old inbred SPF male Donryu rats were inoculatedwith sialodacryoaden it is virus ( SDAV) and ascites hepatoma AH66cells. The effect of a local ized viral infection on tumorigenesisand invasive capacity of transplanted tumor cells was examined. Criteriawere used to compare the invasive capacity of tumor cells and alterationsin immune response of host animals with SDAV-infected or uninfected,inc luded histopathologic, serological, and indirect immunofluorescenceanalysis with cytofluorography of the T cells subset McAb. Based onthese criteria, the tumorigenesis and invasive capacity of the tumor cellswere found nore frequently in tlie lungs of SDAV-infected rats thanuninfected rats.The viral antigens were detected in the salivary glands.respiratory tract, and the lung tissue with tumor nests by immunohisto-chemistry. FACS analysis of lymphocytes isolated from the bloodrevealed that the CD4/CD8 ratio was decreased in injected tumorcells with SDAV-infected-rats,whereas the CD4/CD8 ratio was increasedin injected tumor cells with SDAV-uninfected-rats. Results showed anincrease on the invasive capacity of tumor cells to localized viralin fection and the increased growth rate of implanted asciteshepatoma might be related with the decrease of help T cells ofblood and the mononuclear cells of the lungs due to SDAV-infection.