Mucosal prime-boost SHIV vaccine induces delay in viremia after intrarectal infection of rhesus macaques. Viral load is expressed as viral RNA copies per milliliter plasma (by NASBA assay) versus time after challenge. By the exact Wilcoxon-Mann-Whitney test (StatXact 6), the prime-boost group 2 peak viral titers (tmax) were significantly delayed (P = .006) compared with those of groups 1 and 3, whereas groups 1 and 3 were not significantly different from control group 4. The day (mean ± SE) of tmax peaks for groups 1, 2, 3, and 4 were 18.8 ± 1.7 (median, 17), 38.0 ± 4.04 (median, 38), 21.2 ± 1.72 (median, 24), and 20.5 ± 3.50 (median, 20.5), respectively.