Mycobacterium avium contamination has been described as a putative contaminant of nonphagocytic mammalian cells. Screening of numerous cultured nonphagocytic mammalian cell lines revealed the presence of intracellular bacteria that were identified as M. avium-intracellulare. An extensive and critical analysis of the origin of infection, of cure protocols, and of biological manifestations in M. avium-infected cells is presented. As no tremendous visible alteration of turbidity or pH of cell culture media, and no morphological change occurred in most M. avium-infected cell cultures, detection of an infection by these bacteria is rather difficult. Recommendations are given for treatment of irreplaceable cultures and prevention of mycobacterial contamination in a tissue culture facility.