Study design. Rhesus macaques were randomized into three groups (n = 6 per group). Red arrow, Group 1 received 33C6-IgM intrarectally (i.r.); blue arrow, Group 2 was given 33C6-IgG1 i.r.; empty arrow, Group 3 received only phosphate-buffered saline (PBS) i.r.; black arrow, high-dose i.r. SHIV-1157ipEL-p challenge with 31.5 50% animal infectious doses (31.5 AID50). 33C6-IgM, 33C6-IgG1 or PBS was given 30 min before the virus challenge.