Broadly neutralizing antibodies have shown promise in protecting monkeys against SHIV, a simian version of HIV. Scientists genetically modi--fied two such antibodies to make them stay in the body longer. Then they gave single infusions of one of the two antibodies to rhesus macaques and exposed the animals to SHIV weekly. The animals that received the antibodies contracted SHIV after a median 17 or 27 weeks compared with just three weeks among those that received a placebo infusion. The investigators also combined both antibodies and injected the combo under the skin of six monkeys (as opposed to into a vein) using a dose equal to one third of the infusion. This method protected the animals against weekly SHIV exposures for a median of 20 weeks. An early human trial of the more effective antibody is under way.