The treatment of peritonitis in continuous ambulatory peritoneal dialysis patients is empiric until the bacteriological results are available. The Lymulus amebocyte lysate assay (LAL) is a very sensitive method for the detection of endotoxin, a structural component of gram-negative bacteria. We performed the LAL assay in a prospective study in 36 consecutive episodes of peritonitis. The LAL assay was positive in all 10 episodes of gram-negative peritonitis (100% specificity). Treatment directed specifically against gram-negative or -positive infection was started based on the LAL assay result. In 26 episodes with LAL-negative test, a gram-positive bacterium was cultured in 23 episodes, in 1 there was fungal infection and 2 were sterile. In summary: the LAL assay is a rapid (1 h) and sensitive method for the differentiation of gram-positive or -negative peritonitis and enables starting an immediate and more appropriate antibiotic therapy.