The oncogenic properties of hamster embryo cells transformed by herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) and SV40 virus following photodynamic inactivation using neutral red were determined by subcutaneous inoculation into newborn Syrian hamsters. Cells transformed by all three viruses produced palpable tumors after different latent periods. Histopathological examination showed that HSV-2 tumors were fibrosarcomas and metastases were often seen in the lungs. HSV-2 primary tumors were reinoculated subcutaneously into weanling hamsters; they developed palpable tumors within 2 weeks. HSV-specific antigens were detected in the cytoplasm and/or on the surface of both the HSV-1 and HSV-2 tumor-cell cultures by the indirect immunofluorescence technique. The same method revealed SV40 tumor antigen in the nuclei of the SV40 tumor cells. Sera from HSV or SV40 tumor-bearing hamsters gave positive reactions when tested against HSV-infected hamster cells or SV40-infected monkey cells, respectively. These results demonstrate that herpes simplex virus and SV40, whose infectivity was lost following photodynamic inactivation, retained the virus genetic information necessary for transformation of normal cells to an oncogenic phenotype.