Greater than 10% human CD45+ cells in the peripheral blood is associated with higher susceptibility to HIV-1 transmission. Hu-mice with <10% circulating hCD45+ cells (A–D) and >10% circulating hCD45+ cells (E–H), identified by flow cytometry, were challenged IVAG with low (103) (A, C, E, G) or high (105 TCID50 (NL4.3-BAL)) (B, D, F, H) dose HIV-1. Paired viral titres (RNA copies/mL) were quantified in the vaginal lavage (blue) and plasma (red) of Hu-mice by clinical real-time RT-PCR. Infection did not occur in Hu-mice with <10% hCD45+ cells in the peripheral blood when challenged with low dose HIV-1 (A, C – 1/6 infected) while those challenged at high dose were more likely to be infected (B, D – 8/12 infected). Several Hu-mice with >10% hCD45+ cells in the peripheral blood became infected following IVAG challenge with both low (E, G – 4/5 infected) and high dose (F, H – 10/12 infected) HIV-1. Each symbol represents an individual Hu-mouse and paired titres are denoted by identical symbols on the corresponding graph (ie. A and C, B and D, E and G, F and H). Data was pooled from >3 independent experiments. One experiment did not include vaginal and plasma titres at week 1, thus some values for week 1 titres were omitted. IVAG: intravaginal.