The question of specificity of Limulus amebocyte lysate (LAL) test in the diagnosis of endotoxemia has been a limiting factor of its clinical application. Using a femtogram-sensitive spectrophotometric LAL assay 35 of 36 septic postoperative patients showed an excellent correlation (almost 100%) between positive LAL tests and culture-proven gram-negative bacteremia. Twenty patients of this group demonstrated a significant correlation (p < 0.05) between elevated total white blood cell counts and Escherichia coli Endotoxin equivalents (EcEe). All 22 liver cirrhotic patients with potentially enteric endotoxemia yielded LAL positive reactions (100%) but 9 of 22 correlated significantly as to leucocytosis and elevated EcEe (2 p < 0.01). A significant correlation between EcEe and leucocyte counts was found for all neonates and all parturients respectively (2 p < 0.001), 2 p < 0.01). In vitro tests showed that leucocytes gave positive LAL tests. Supernates of a Ficoll sedimented crude leucocyte preparations, induced LAL positivity in a dose dependent manner, while all reagents per se used in the preparation, were negative. This study suggests that heat-labile factors from leucocytes apart from endotoxin are responsible for positive LAL reactions.