The human promyelocytic HL-60 cell line can be induced to differentiate to neutrophil-like cells in response to a variety of chemical stimuli. We have found that retinoic acid-treatment of HL-60 cells over a period of 6–8 days resulted in a progressive increase in the proportion of cells with mature neutrophil morphologies and was closely followed by an increase in the proportion of cells exhibiting the morphological characteristics of apoptosis, the non-pathological mode of cell death. Using Percoll step-density gradients we have demonstrated a marked increase in the buoyant density of these cells and have used this density difference to obtain enriched fractions of cells for more detailed study. Degradation of the nuclear DNA of these cells into integer multiples of about 200 base pairs, indicative of endogenous endonuclease activation a major characteristic of programmed cell death, was also demonstrated. From these observations we conclude that the mode of cell death in cultures of terminally differentiated HL-60 cells is that of apoptosis. These results parallel those of a recent report which has shown apoptosis to be the mode of cell death of ageing peripheral blood neutrophils. Because of this, we believe that our observations further validate the use of the HL-60 cell line as a model system for the study of human granulopoiesis in vitro and further, that this model system may be useful for gaining insight into the underlying mechanisms involved in apoptosis.