Protection from Infection by HIV-1 in Vivo. We have previously shown that NNY-RANTES was superior to AOP-RANTES in protecting SCID mice grafted with human peripheral blood lymphocytes (hu-PBL-SCID mice) from infection by HIV-1 (17). Protection was achieved by treating mice with a single bolus injection of 1 mg of NNY-RANTES just before HIV-1 challenge and providing continuous low-dose administration of NNY-RANTES by osmotic pumps. Here we report that a single injection of 500 μg of NNY-RANTES was unable to protect against infection, whereas a single injection of PSC-RANTES at either 500 μg or 150 μg protected all of the challenged mice against HIV-1 infection. Injection of 50 μg was partially protective (Fig. 4). The PSC-RANTES was in no way formulated for bioavailability, and we note that the continuous infusion experiments mentioned above suggested that an equilibrium concentration of NNY-RANTES between 75 and 96 pM was sufficient to sustain protection against infection (ref. 17; D.M., unpublished results).