The Limulus amoebocyte lysate test was used to detect the presence of endotoxin in the venous blood of 10 patients following urethral instrumentation. The test was negative in all patients before instrumentation, became positive in 9 immediately after instrumentation and in 1 30 min after instrumentation. In 6 patients endotoxin reappeared between 1.5 h and 4.5 h after instrumentation. All blood samples, cultured aerobically and anaerobically, were sterile. Escherichia coli were cultured from one midstream specimen of urine and from one urethral swab in the same patient taken before instrumentation. There were no significant changes in either pulse rate or axillary temperature following instrumentation.