In 473 consecutive febrile patients a sensitive and rapid chromogenic limulus assay was used to assess the value of endotoxaemia versus bacteraemia for predicting development of the syndrome of septicaemia. In each patient three blood specimens for culture and endotoxin testing were obtained at the onset of fever. Blood pressure, urinary output, and the occurrence of thrombocytopenia and metabolic acidosis were recorded prospectively during three days of follow-up. Septicaemia developed in 19 patients (4%). The sensitivity, specificity, and likelihood ratio for a positive result with the endotoxin assay were 79%, 96%, and 20, respectively. The corresponding indices for bacteraemia were 89%, 78%, and only 4. The results suggest that endotoxaemia is a clinically valid indicator for impending gram-negative septicaemia (positive predictive value 48%) and that the absence of endotoxaemia virtually rules out the risk that septicaemia will ensue (negative predictive value 99%).