CD28 expression by donor T cells affects the degree of donor chimerism after transplantation of B6 splenocytes into unirradiated BDF1 mice. Two and 15 days after injection of donor cells, splenocytes from normal BDF1 controls and from BDF1 recipients of CD28^+/+ or CD28^−/− donor cells were stained for recipient-specific (H2D^d) class-I antigen and for CD4, CD8, or B220 and analyzed by two-color flow cytometry. Results were similar with three mice in each group.