The leucocyte alkaline phosphatase activity in mature neutrophils of different ages.

The marrow myeloid precursor cells of a haematologically normal patient were labelled by intravenous injection of tritiated thymidine. Young labelled segmented neutrophils were then released from the marrow into the blood by an injection of cortisol. These PMN had a significantly lower LAP activity than the older blood neutrophils. It is shown that within the morphologic boundaries of the segmented PMN, the cells are still in different stages of cytoplasmic maturation. In addition, labelled and unlabelled neutrophils showed a linear and parallel increase of LAP activity during the 24 h following cortisol injection. This observation suggests that the neutrophil prematurely released in the blood can mature into normal LAP=PMN and more generally that LAP activity of a blood neutrophil increases with time.