A newly developed modification of the limulus amebocyte lysate test for quantification of endotoxin levels in blood is described. The chromogenic peptide carbobenzoxy-Gly-Gly-Arg-4-methyl-cumarinyl-7-arnid proved to be most suitable. The liberated fluorescent dye is diazotized with N(1-naphtyl-)-ethylen-diamin-dihydrochloride. Using this statistically proved reliable and sensitive test, endotoxin serum levels of healthy persons and patients undergoing major surgical treatment were compared. In the postoperative phase endotoxin serum levels up to 0.5 ng/ml can be detected without clinical signs of septicemia. Healthy persons show endotoxin serum levels up to 0.08 ng/ml. In rats no difference of endotoxin serum levels was detected in the portal vein, and in arterial and venous blood. So a physiological endotoxin resorption from the intestine followed by a clearance during the liver passage seems to be doubtful in this species.