Cell-free broth filtrates of a strain of Klebsiella pneumoniae serotype 5 retained their capacity to induce fluid secretion in the rabbit ileal loop model after heating (100 C for 30 min), acid treatment (pH 4.4), and dialysis through viscose tubing. Sequential passage of an acetone precipitate of the broth filtrate through Diaflo membranes yielded an active fraction in the UM 2, but not the UM 10, retentate; this observation indicates that the molecular weight of the enterotoxin is in the range of 1,000-10,000. Klebsiella pneumoniae enterotoxin thus resembles the heat-stable enterotoxin of Escherichia coli in a number of respects.