Human teratocarcinoma cells cultured in vitro can be induced to produce retrovirus-like particles. The induction procedures are the same as those previously shown to induce the synthesis of animal retroviruses. Electron microscopical evidence is presented that the human teratocarcinoma-derived (HTD) particles are most closely related to the type C retrovirus strains. HTD particles can be banded at 1.16 g/ml in linear sucrose gradients, the characteristic density for retroviruses, and subsequently be used for negative staining and fine structure analysis.