Type-C virus particles were revealed by electron microscope in 6 of 9 tumours of cultured and biopsied human cancers heterotransplanted into nude mice. Some tumours in nude mice were explanted for in vitro cultivation. The virus particles were also found in the cultures derived from the virus-positive tumors. They were mostly found extracellularly, but the particles in budding process were also encountered frequently. Cytological study and karyotype analysis of the cultured cells proved these virus-releasing cells as of hus of human origin. From the close correlation between the statistical virus counts and the complement fixation titers for murine gs antigen of the tumors and their cultures, these viruses propagated in human cancer cells were confirmed to be infectious viruses of nude mouse origin. The virus replicating in human cancer cells was readily infected in some of innocent human cancer cells by co-cultivation. It is to be emphasized that infection of animal endogenous viruses on heterotransplanted human cancer cells is a bothersome contamination for human cancer research, especially when searching for a human tumor virus candidate.