We have constructed and we propose to characterize a replication defective version of the SIVsm genome to use as a model single cycle vaccine candidate. The viral genome has been modified with multiple mutations in three genes such that one round of infection can occur with the subsequent expression of all of the viral genes, including production of virus particles, but with no virus spread. We have mutated the viral vif, env, and nef genes with sixty-five point mutations and two codon deletions to ablate multiple functional domains while allowing expression of potential CTL epitopes. These lost functions will be complemented during vaccine production to permit transient infectiousness. Expression of viral genes and transient infectiousness will be documented in cell culture. This single cycle vaccine candidate will then be used to infect a group of six macaques to permit an initial immunological characterization of vaccine efficacy. After a prime and two boosts, using viruses pseudotyped with different envelope proteins, the animals will be challenged with SIVsm E660 and monitored for one year. Finally, we will examine inactivating mutations in the SIVsm Env protein that permit high level surface expression as a step toward designing a vector that can elicit both CTL and neutralizing antibody responses. [unreadable] [unreadable]