Reconstitution of myeloablated mice after secondary transfer of BM cells obtained from mice that had previously been grafted with cultured cells. Mice that had received a transplant with either catalase-cultured cells or control cells were killed 30 weeks after transplantation and BM cells prepared. Cells from each group of mice were pooled and one million viable BM cells were transplanted into irradiated C57Bl/6 (Ly5.2) recipients as previously described. After 1 month the recipients were bled and the proportion of culture-derived (Ly5.1) cells in the peripheral blood of each mouse was determined. The difference between the proportion of cultured cells recovered from the catalase-treated and control cultures after the injection of 50 000 cells is significant with a P value of less than .001 (Student t test; 2-tailed, unpaired samples). ⋄ indicates 50 000 cells from control culture; ▵, 20 000 cells from control culture; ♦, 50 000 cells from catalase culture; ▴, 20 000 cells from catalase culture; •, background; and —, mean.