Earlier studies reported that certain large hydrophobic poloxamer surfactants were able to inhibit the growth of Mycobacterium avium-M. intracellulare complex (MAI) in broth and to produce synergistic enhancement of the activity of rifampin. CRL-1072 was synthesized to have an optimal structure for antimicrobic effects and greater purity. Its MIC for MAI in broth was greater than 100 µg/ml. Surprisingly, its MIC for MAI growing in human U937 monocytoid cells was much lower, 5 µg/ml. A still lower concentration, 0.1 µg/ml, produced synergistic enhancement of the activities of clarithromycin, rifampin, amikacin, streptomycin, and clindamycin, but not isoniazid, against MAI infecting monocytoid cells. Mice tolerated injection of doses of CRL-1072 as high as 125 mg/kg of body weight. Pharmacokinetic analysis revealed that the copolymer had an elimination half-life of 60 h and suggested dosing regimens that might produce therapeutic concentrations in tissue. In a mouse model of acute MAI infection, CRL-1072 significantly enhanced the bactericidal activities of clarithromycin and rifampin when it was administered at 1.0 mg/kg intravenously (i.v.) three times per week. CRL-1072 given i.v. or orally also enhanced the bactericidal activity of clindamycin against MAI.