Dose-dependent effects of estradiol, progesterone, or both in combination on spontaneous neutrophil apoptosis. Neutrophils (1 × 10⁶ cells/mL) isolated from male (▪; n = 10) and female (♦; n = 10) controls were incubated in vitro with increasing physiologic doses (10^-12 to 10^-7 g/mL) of estradiol (A), progesterone (B), or both in combination (C). Propidium iodide DNA-binding staining using flow cytometry assessed neutrophil apoptosis at 24 hours. *P < .05 versus control (Con). Error bars indicate mean ± SD.