Apoptosis of polymorphonuclear leukocytes (PMNs) is a critical step in the resolution of tissue inflammation. PMN apoptosis has been studied extensively in vitro, and diverse inflammatory mediators have been shown to modulate the process. The reported effects of interleukin-6 (IL-6) on PMN apoptosis are inconsistent; however, analysis of published studies reveals at least one discriminating factor--the use of varied concentrations of PMNs in the experimental design. Consequently, we hypothesized that the in vitro effects of IL-6 on PMN apoptosis varied with the concentration of PMNs in culture. PMNs isolated from healthy human donors were cultured at concentrations from 1 to 20 x 10(6)/mL, and incubated with IL-6 doses from 1 to 100 ng/mL. PMNs cultured at 1-5 x 10(6)/mL were unaffected by IL-6; in contrast, IL-6 inhibited apoptosis in PMNs cultured at 10-20 x 10(6)/mL, compared with untreated similarly concentrated PMNs. These data suggest caution in interpreting in vitro studies of apoptosis; on the other hand, appropriately designed experiments may help elucidate the regulation of apoptosis in vivo.