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SUMMARYThe morphology and development of simian haemorrhagic fever virus was studied in tissue cultures infected with low multiplicities of virus. Early cytoplasmic lesions which were observed 24 hr after infection consisted of increased concentrations of polysomes and small vesicles IOO nm. in diameter filled with densely staining fibrillar material. As the lesions progressed, thickened membranous structures developed at the periphery of the vesicles. The membranes appeared to lengthen and thicken by a process of end-to-end and side-to-side fusion. Serial sections demonstrated that the fused, thickened structures were lamellae. Their role in the morphogenesis of simian haemorrhagic fever virus is not presently known.Spherical virus particles measuring 4o to 45 nm. in diameter appeared in cells by 72 hr after infection. Infected cells contained vacuoles which enclosed accumulations of enveloped spherical particles. Some particles appeared to be budding into vacuoles.

In rhesus monkeys a wide dosage range of 17D yellow fever (YF) vaccine extending to a level even below that recommended for vaccination of man elicited an immune response providing solid protection to challenge with virulent YF virus. Forty-three of 45 monkeys vaccinated with 10 2.3 or greater weanling mouse mean lethal doses of 17D vaccine were resistant to challenge 20 weeks later with virulent Asibi strain YF virus. Monkeys given graded doses of lesser amounts of vaccine were progressively more susceptible to challenge. With a vaccine dose ≥ 10 2.3 weanling mouse mean lethal doses, plaque neutralization (PN) seroconversion rates were 90% or greater, whereas hemagglutination-inhibiting (HI) and complement-fixing (CF) seroconversion rates were unrelated to vaccine dosage and were generally in the range of 20 to 80%. Ninety-six percent (51 of 54) of immune monkeys had PN titers ≥0.7 log 10 (fivefold) neutralization index as compared to approximately 55 to 65% who showed HI or CF titers ≥2 log 2 (fourfold) neutralization index. After challenge with Asibi strain YF virus, antibody titers of all three tests increaed equally. In rhesus monkeys PN antibody titers were well correlated with YF immunity, whereas HI and CF antibody titers were not.