Nondepleting anti-CD4–treated mice remain immune competent. (A) C57Bl/6 mice were treated as described previously with anti-CD4, but using FVIII-alum or OVA-alum as the tolerizing antigen administered at the time of mAb treatment. Five weeks following the last mAb infusion, the mice were exposed to the same or to the other antigen. Control groups were not treated with anti-CD4. (B) Some mice were immunized with hFVIII-alum on days 50 and 64, and serum hFVIII-specific IgG1 was quantified at day 73. We found that unlike mice initially treated with OVA-alum plus anti-CD4, mice treated with hFVIII-alum plus anti-CD4 remained unresponsive to the subsequent immunization with hFVIII-alum (n = 4; *P < .05). (C) Other mice were immunized at days 50 and 64 with OVA-alum, and serum OVA-specific IgG1 was quantified on day 73. Animals initially treated with hFVIII-alum plus anti-CD4 remained competent to respond to immunization with OVA-alum, unlike mice initially treated with OVA-alum plus anti-CD4 that remained tolerant to OVA (n = 4; *P < .05). Data are representative of 2 independent experiments.