Gross and microscopic lesions of Bolivian hemorrhagic fever (BHF) are described in 10 rhesus monkeys that survived from 30 to 78 days after subcutaneous inoculation with a dose of 10(3) plaque-forming units (PFU) of Machupo virus, a dose which produces a severe and generally fatal disease. Six of the monkeys had been given low doses of homologous immune globulin when initial signs of infection appeared. Monkeys exhibited clinical signs in two phases. The initial signs of acute infection which began to appear about 1 week following inoculation included: diarrhea, depression, anorexia, dehydration, and skin rash. The survivors of this early phase of the illness usually showed improvement before relapsing into the second (or chronic) phase, which was characterized clinically by central nervous system disturbances including incoordination, tremors, convulsions, paresis, and muscle atrophy. Microscopic lesions were similar in both immune globulin-treated and untreated animals. These included widespread lymphoreticular infiltrates in the walls and adventitia of blood vessels of the brain, spinal cord, pancreas, intestine, liver kidney, adrenal, parathyroid, heart, and skeletal muscle. Diffuse lymphocytic infiltrates not confined to the vascular or perivascular tissues were present to a variable degree in many of these and other organs. Several monkeys exhibited lymphocytic inflammation of the choroid, meninges, peripheral nerves, and ganglia.