Serum effect on the activity of a CPPÀAO conjugates is an important issue when considering potential in vivo applications. X/B-containing conjugates were still active in 60% serum while oligoarginine conjugates were not. The greater stability of the X/B-containing conjugates to serum enzymes is likely a factor contributing to their high activity. The loss of activity in high serum concentrations makes oligoarginine CPPs undesirable as potential therapeutic AO carriers.