Phenotypic assessment of the percentage Fas⁺ cells indicates that marked augmentation of Fas antigen expression present in the Peyer's patch cells from C3H/HeN mice is confined primarily to the CLP mouse B220⁺ (B lymphocytes) subpopulation but not the B220⁻. This occurred despite the lack of significant change in the Fas antigen mean channel fluorescence of B220⁺ cells. Significance indicated by * at P < .05 versus sham, Mann-Whitney Utest; mean ± SEM; n = 6 mice sampled/group. Although not shown, Peyer's patch cells stained with antibody to CD4 (T_helper-cell) showed no marked change in the percentage of Fas⁺ cells or their mean channel fluorescence.