The frequency of CD4⁺ T cells giving rise to CFSE^- CD4⁺ T cells is strongly diminished in the absence of MHC class II molecule expression. Various amounts (10³to 5 × 10⁶) of CFSE-labeled lymph node CD4⁺ T cells from normal C57BL6 mice were injected intravenously into CD3ϵ^-/-mice and CD3ϵ^-/- MHC II^Δ/Δ mice. Twenty-eight days after transfer, lymph nodes and spleen were recovered and pooled, and single-cell suspensions were prepared. (A) Absolute numbers of recovered CFSE⁺ CD4⁺ TCR^hicells are shown as a function of the number of injected cells. (B) Absolute numbers of recovered CFSE^- CD4⁺ TCR^hi cells are shown as a function of the number of injected cells.