Inability of β₁^Δ/Δ mice to respond to hemolytic stress is largely cell autonomous. Lethally irradiated wild-type recipients were reconstituted with β₁^Δ/Δ or β₁⁺ BM cells and after reconstitution were treated with PHZ to induce acute hemolytic anemia. Six days after the first PHZ injection, mice were killed, and cellularity, progenitor content, and erythroid differentiation (number of TER119/CD71⁺ cells in gates R1 [CD71^hi/TER119^lo], R2 [CD71^hi/TER119^hi], and R3 [CD71^lo/TER119^hi]) were assessed in the spleen (top panel) and BM (middle panel). Transplanted (Tx) mice (ctrl, n = 6; and β₁^Δ/Δ, n = 6) were compared with corresponding nontransplanted counterparts. *Significant difference over the respective β₁⁺ control group. Cross-hatched bars represent transplanted; and white bars, nontransplanted. P < .05. #Significant difference over nontransplanted counterparts (P < .05). Note that transplanted β₁^Δ/Δ mice (black bars) have a similar response to PHZ as nontransplanted β₁^Δ/Δ mice (gray bars). The bottom panel shows the difference in spleen sizes of 2 control-transplanted mice (+/+ recipients transplanted with +/+ donor cells) after PHZ treatment at day 6 (left), compared with 2 β₁^Δ/Δ-transplanted mice (+/+ recipients transplanted with β₁^Δ/Δ cells, right). Spleens were photographed with a Nikon Coolpix 995 camera next to a ruler measuring millimeters.