Both complement (C) and neutrophils (PMN) are activated in critically ill patients. To evaluate the role of endotoxin in this response, we studied C activation products and PMN cell surface receptors in seven normal subjects before and after endotoxin (USRef 20 U/kg) or saline solution administered on separate occasions. By 4 hours, with endotoxin only, all subjects had myalgia, headache, an increase in body temperature and heart rate, and leukocytosis that returned to normal by 24 hours. At the same time, PMN cell surface receptors for the complement opsonin C3b increased, as measured by indirect immunofluorescence, rising to 251 +/- 44% of baseline by 4 hours (p less than 0.01) and remaining elevated at 24 hours (237 +/- 16%, p less than 0.01). PMN receptors for iC3b increased to 308 +/- 49% of baseline by 4 hours (p less than 0.02) and returned to normal by 24 hours. There was no change in plasma of C3a desArg, C4a desArg, and C5a desArg (4 hours: mean C3a: 153.4 +/- 11.5 ng/ml versus 176.2 +/- 16.2 ng/ml for saline solution, p = ns; C4a: 159.6 +/- 32 ng/ml versus 151.4 +/- 21 ng/ml, p = ns; C5a: undetectable). To confirm the lack of C activation, we examined PMN chemotaxis (CTX) to C5a for any impairment caused by prior in vivo exposure to C5a. CTX to C5a was unaffected (4 hours: 109% +/- 22% of normal versus 114% +/- 10% for saline solution, p = ns). PMN CTX to formyl-methionyl-leucine-phenylalanine and PMN phagocytosis and killing of S. aureus were also unaffected by endotoxin. Thus, a single dose of endotoxin produced a subjective febrile illness and precipitated sustained PMN activation as indicated by increased PMN cell surface complement receptor number in the absence of C activation.