Nicotine exposure during differentiation does not influence the ultrastructural characteristics of HL-60 cells. Promyelocytic HL-60 cells (A, ×1550) contain a large or oval nucleus (N) with diffuse chromatin, a prominent nucleolus (nu), and a variable number of large cytoplasmic vacuoles (V) some of which contain variably dense cores (d); Five-day, DMSO-differentiated HL-60 cells without nicotine treatment (B, ×2650); or exposed to 10-6 M nicotine (C, ×4300); or 10-4 M nicotine (D, ×2650) contain a markedly indented and segmented nucleus with pyknotic nuclear chromatin (N). The nucleolus has disappeared. Small uniformly dense granules (dg) as well as some large vacuoles with variably dense cores (dc) were found in the DMSO-differentiated cells. M: mitochondria. No qualitative changes to neutrophil ultrastructure could be ascribed to nicotine exposure during HL-60 differentiation. The bars represent 1 μm.