The in vivo fate of amphotropic murine leukemia retrovirus was studied in five rhesus monkeys. Retrovirus infused intravenously into 3 normal animals and 1 immunosuppressed animal was cleared rapidly from the circulation and subsequent viremia has not been detected (mean follow-up of 27.4 months). A fifth monkey was immunosuppressed and transplanted with virus-producing autologous fibroblasts in addition to an intraperitoneal injection of virus. This animal was viremic for 2 days and its lymph node cells and peripheral blood mononuclear cells were shown to be producing virus for up to 22 days post-inoculation, but subsequently has been negative after 17.0 months of analysis. In the 5 animals studied (combined mean follow-up of 25.7 months), clinical illness has not been identified at any time. Therefore, murine amphotropic retroviruses do not appear to pose an acute health risk.