Cell-free preparations of the whole-cell lysate and ultrafiltration (UF) fractions of broth cultures of a strain ofEnterobacter cloacae, isolated from a Puerto Rican with tropical sprue, were assayed for their ability to induce in vivo net water secretion in the rat jejunum. The whole-cell lysate and UM-10 retentate of broth cultures were inactive. The UM-2 retentate and filtrate were active at a concentration of 100 ,g/ml or more; the toxigenic activity was entirely retained, and increased to

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... ,g/ml, by a UM-05 membrane; washing this retentate yielded a fraction with an activity of 10 ng/ml. Stationary aerobic culture conditions yielded the most active UF fractions when ammonium sulfate was used as the precipitating agent, whereas anaerobic culture conditions produced the most active fractions in broth cultures precipitated by acetone. Passage of the active acetone-precipitated UF fractions through a Sephadex G-25 column yielded eluate pools with enhanced toxigenic activity in, or adjacent to, the void volume, but maximum activity of the ammonium sulfate-precipitated UM-05 retentate eluated at a Ka, of 0.38 to 0.52. Neither of the most active gel filtration elution INFECT. IMMUN. on May 8, 2020 by guest http://iai.asm.org/ Downloaded from