We evaluated harvested marrow cells for total nucleated cells (27.49 x 10(9)), absolute 'lymphocyte' count (6.29 x 10(9)) and CD 34 positive cells (3.57 x 10(9)). The same parameters were studied after in vitro manipulation to remove RBCs and plasma. Reinfused WBCs contained 12.87 x 10(9) nucleated cells, 4.25 x 10(9) absolute 'lymphocyutes' and 3.34 x 10(9) CD 34 positive cells. The corresponding figures for loss during in vitro manipulation (tubing, RBCs and plasma) are 14.62 (53.18%), 2.04 (32.43%) and 0.23 x 10(9) (6.44%) cells respectively. Therefore CD 34 positivity may be a better indicator of total yield, loss during manipulation and reinfusion of hemopoietic progenitor cells in bone marrow transplantation.