We examined endotoxins and limulus amebocyte lysate-reactive material (LAL-RM) in serum from 87 patients on hemodialysis, using the conventional chromogenic limulus test (CCLT) and a new specific endotoxin assay with factor G-free LAL (endotoxin-specific test: EST). All patients with regenerated cellulose dialyzers had increased CCLT values, whereas the EST values were not higher than in controls. This discrepancy can be explained by the LAL-RM which is cellulose-derived and cross-reacts with LAL via factor G. Using EST for measurements of endotoxin, 6 patients out of 87 had pathological endotoxemia and all of these patients were associated with either cirrhosis, infection, or malignancy. Some patients who had no comlications showed fever before or during dialysis, but they did not have high endotoxin levels. EST is useful for the diagnosis of endotoxemia in patients on hemodialysis because of the presence of LAL-RM in serum. Endotoxemia is rare in patients on hemodialysis, if they are not associated with infection, cirrhosis, or other complications.