Trauma patients develop a severe immunosuppression that includes suppression of natural killer (NK) cell activity although numbers of NK cells are not reduced. The mechanism of suppression of NK cell activity after major trauma is not known. The aim of the present study was to investigate the in vitro effect of plasma samples from trauma patients (TP) on the cytotoxic activity of normal NK cells. Buffycoat mononuclear cells (5x10(5)/well) were preincubated with either TP or plasma samples from age and sex matched healthy controls (CP) for 0, 16 or 40 h. These effector cells were then cultured with 51Cr labeled K-562 cells (2x10(4)/well) for 4 h at 37 degrees C and % lysis was calculated. No significant differences in % lysis between CP and TP were found with 0 or 16 h preincubation, however 40 h preincubation with TP severely suppressed NK cell function (p=0.003) as compared to preincubation with CP for the same period. Addition of neutralizing anti-IL-4, anti-TGF-beta1, or anti-IL-10 antibodies did not reverse the NK cell suppression. There was a partial reversal of NK cell suppression by catalase but not by SOD or L-NMMA. Removal of monocytes from buffycoat mononuclear cells also significantly reversed the NK cell suppression. These data suggest that suppression of NK cell activity in trauma patients may be an accessory cell dependent phenomenon and may partially depend on production of reactive oxygen metabolites (ROM).