(BALB × B6) F1: Effects of DR and age on in vivo functional abilities of BMCs. (A) Total marrow functional ability based on the average composition of erythrocytes and lymphocytes. (B) Total marrow functional ability expressed in repopulating units. Assays were performed as described in the legend to Figure 1, except that recipients and donors were (BALB × B6) F1 hybrids with Gpi1^a/b and Gpi1^a competitors. A total of 3 to 5 donors was used for each type. Old versus young were significantly different (P < .01). Error bars represent SE.