Retroviruses received widespread attention in the past not only because they caused tumors in animals and, perhaps, in man, but also because they served as useful tools to elucidate molecular mechanisms involved in the control of eukaryotic gene expression. In this brief overview, evidences are presented that retrovirus-like particles can regularly be demonstrated in human teratocarcinomas cultured in vitro. These viruses are morphologically reminiscent of animal retrovirus strains but show also unique structural features. The viruses possess endogenous reverse transcriptase activity and can be banded at 1.16 g/ml in sucrose gradients, a density characteristic of retroviruses. They can clearly be distinguished from animal retrovirus strains on immunological grounds. We will also briefly summarize the data accumulating for the human T-cell lymphoma viruses (HTLV) which were recently discovered first by Gallo et al. and independently by Hinuma and Miyoshi. HTLV seem to play an etiological role in the establishment of human adult T-cell leukemia/lymphoma and thus represent the first pathogenic human retroviruses.