A rise in the percentage of large unstained cells in the peripheral blood determined by the Hemalog D90 automated differential counter is a feature of impending myeloid engraftment following bone marrow transplantation.

The early phase of bone marrow regeneration has been monitored by automated differential counts on the peripheral blood using a Hemalog D90, following 107 transplant procedures. An elevated percentage of large unstained cells (LUCs) was detected in over 98% of cases and in 72% of cases the rise in percentage LUCs preceded the rise of the total leucocyte count into the detectable range on a Coulter counter (greater than 0.3 X 10(9)/1) by an average of 4 days. These LUCs are shown to be CD8 + T lymphocytes. The ability to detect the earliest signs of regeneration is particularly useful when regeneration is delayed and repeat marrow infusion is considered.