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To define the ratio of simian immunodeficiency virus (SIV) RNA molecules to infectious virions in plasma, a ramp-up-stage plasma pool was made from the earliest viral RNA (vRNA)-positive plasma samples (collected approximately 7 days after inoculation) from seven macaques, and a set-point-stage plasma pool was made from plasma samples collected 10 to 16 weeks after peak viremia from seven macaques; vRNA levels in these plasma pools were determined, and serial 10-fold dilutions containing 1 to 1,500 vRNA copies/ml were made. Intravenous (i.v.) inoculation of a 1-ml aliquot of diluted ramp-up-stage plasma containing 20 vRNA copies infected 2 of 2 rhesus macaques, while for the set-point-stage plasma, i.v. inoculation with 1,500 vRNA copies was needed to transmit infection. Further, when the heat-inactivated set-point-stage plasma pool was mixed with ramp-up-stage virions, infection of inoculated …