Profiles of internalization and replication of Mycobacterium tuberculosis (MTB) or M. avium complex (MAC) within A-549 human type II alveolar epithelial cell line (A-549 cells) were studied and the results were compared with the mode of internalizing and proliferative behaviors of the organisms within murine peritoneal macrophages (M phi s) and various M phi-like cell lines (murine J774A. 1, human THP-1, and human MONO-MAC-6). First, MTB and MAC internalized not only in peritoneal M phi s and M phi-like cell lines but also in A-549 cells. Secondly, MTB and MAC replicated within A-549 cells and these organisms displayed much more vigorous intracellular multiplication in A-549 cells than in murine peritoneal M phi s and J774A. 1 M phi-like cell line. Human M phi-like cell lines (THP-1, MONO-MAC-6) allowed the growth of MTB and MAC equally or occasionally more rapidly, as compared to the case of A-549 cells.