The dynamics of pathophysiological and immunological parameters monitored in monkeys Papio hamadryas infected with the guinea pig-adapted Ebola virus strain demonstrated that this viral strain preserved its virulence for monkeys and caused the disease with characteristic features similar to those caused by non-adapted Ebola virus. However, certain previously unknown patterns have been observed: (1) prolongation of the febrile period by two days; (2) extended period was characterized by stability of serum biochemical parameters; (3) marked vacuolization of the neutrophil cytoplasm; (4) appearance of juvenile lymphocytes on day 3 and by the end of the disease; and (5) a considerable increase in the spontaneous mononuclear proliferation (along with a decrease in the mitogen-induced proliferation) during the terminal stage of infection. The severity of pathological coagulation was found to correlate with the activity of serum cytokines IFN-alpha and TNF-alpha: their activities increased about 250- and 100-fold, respectively. There was significant alteration in the activity of natural killer cells, that dropped by the time of animal death.