Abstract Anti-CD4 has been extensively studied in murine models of autoimmunity and transplantation. The timing of anti-CD4 administration in these systems is critical because anti-CD4 effectively blocks primary T-dependent responses but does not diminish ongoing or memory responses in immunized animals. These differential effects suggest that anti-CD4 suppresses a subpopulation of CD4+ cells. We previously observed in vitro that simultaneous activation through TCR-T3 rescued CD4+ cells from anti-CD4 elimination. From this we hypothesized that activated CD4+ cells resisted the effects of anti-CD4. We now show that in vivo treatment with anti-CD4 preferentially eliminated resting, naive CD4+ cells rather than memory and effector CD4+ cells. The CD4+ cells that remained after anti-CD4 treatment exhibited evidence of recent activation, because a higher percentage expressed IL-2R, regardless of subset phenotype. Moreover, Mls-1-primed, anti-CD4-treated mice showed a higher percentage of V beta 6+ (Mls-1 reactive) CD4+ cells than either unprimed mice, anti-CD4-treated mice, or Mls-1-primed controls, implicating the importance of recent activation. These anti-CD4-resistant cells also retained their functional abilities. T cells from BALB/c mice treated with anti-CD4 after Mls-1 immunization maintained their MLR proliferation against DBA/2 stimulator cells. In addition, anti-CD4 did not reduce T-dependent antibody responses in mice previously primed against the Ag cholera toxin or SRBC. Thus, activated CD4+ cells resist the suppressive effects of anti-CD4. Our findings have critical implications for the ongoing clinical trials using anti-CD4.