The Limulus amoebocyte lysate endotoxin assay was evaluated as a method for rapid diagnosis of acute bacterial meningitis in a series of 305 patients. The results of Limulus assays on cerebrospinal fluid (CSF) samples from these patients were compared with the results for each patient of routine bacterial cultures and Gram stains. Positive Limulus tests were obtained on initial CSF specimens from 84% of patients with culture-proven bacterial meningitis, including all patients with meningitis due to gram-negative organisms. Initial Gram-stained smears revealed the presence of organisms in 68% of the patients. One patient with pneumococcal meningitis had a weakly positive Limulus assay, whereas patients with meningitis due to other gram-positive organisms, those with aseptic meningitis, or patients without meningitis had negative CSF Limulus tests. The Limulus assay also demonstrated the persistence of endotoxin in the CSF of certain patients during antibiotic therapy, especially patients with Haemophilus influenzae meningitis. The Limulus test proved to be a rapid, reliable indicator of the presence of gram-negative organisms in the CSF of patients suspected of acute bacterial meningitis.