We developed a rapid, quantitative culture method to estimate the replication of Mycobacterium avium-intracellulare complex (MAIC) in human peripheral blood mononuclear cells. Mononuclear cells were plated in a 96-well tray, infected with clinically isolated strains of MAIC in the presence of autologous plasma, and further cultivated for 1 to 2 weeks in a tissue culture medium. No MAIC cells proliferated extracellularly, since human plasma inhibited extracellular growth of the mycobacteria. The mononuclear cells were lysed through a brief treatment with alkali, and surviving intracellular mycobacteria were diluted and plated with tissue culture medium in a 96-well tray. Mycobacterial colonies were counted under a microscope after a 5-day incubation. The number of viable MAIC cells continuously increased, reaching 10 times the number of inoculated cells in a week. Thus, mononuclear phagocytes were the permissive site for the replication of MAIC. Intra- and extracellular susceptibilities of seven MAIC strains to four aminoglycoside antibiotics were then studied. The mycobacteria were most susceptible in vitro to dibekacin (MICs, 3.13 to 12.5 micrograms/ml). Dibekacin at 12.5 micrograms/ml was bacteriostatic to five of seven strains in the monocytes. Also, intracellular replication of the other two strains was greatly suppressed by that concentration of dibekacin.