The hemorrheological determinants plasma fibrinogen, plasma viscosity, red cell aggregation, and hematocrit were studied in 11 patients before and 4 and 24 hr after a 15 g immunoglobulin infusion to assess the effect of intravenous immunoglobulin administration on blood rheology in septic shock. Four hr after the immunoglobulin administration, plasma fibrinogen decreased significantly [472 (175-950) vs. 522 (182-1,050) mg/dl before administration; median (range); P < 0.01]; after 24 hr, plasma fibrinogen tended to increase again [501 (185-980); n.s.]. No changes were seen in plasma viscosity, red cell aggregation, hematocrit, and coagulation status 4 and 24 hr after immunoglobulin administration. In patients with septic shock, intravenous immunoglobulin administration leads to a decrease in plasma fibrinogen, whereas plasma viscosity and red cell aggregation remain unaffected. The explanation for the decrease in fibrinogen remains speculative. (C) 1993 Wiley-Liss, Inc.