To explore the ability of tubercle bacilli to invade and survive within non-phagocytic cells, we used in this study a human fibroblast cell line, WI-38, derived from normal embryonic lung and a human epithelial cell line, SQ-5, derived from lung squamous cell carcinoma. Live M. tuberculosis Erdman and M. tuberculosis H37Rv invaded WI-38 cells more efficiently than live M. tuberculosis H37Ra, M. bovis Ravenel, M. bovis BCG Tokyo and M. bovis BCG Pasteur. The capability of tubercle bacilli to invade WI-38 cells was Erdman > or = H37Rv > BCG Pasteur [symbol: see text] M. bovis Ravenel [symbol: see text] BCG Tokyo > H37Ra. A similar invasive ability was observed using SQ-5 cells. In contrast with live bacilli, heat-killed bacilli failed to invade WI-38 cells, whereas they were detected within SQ-5 cells. These results and incorporation of latex beads suggest that SQ-5 cells, but not WI-38 cells, possess phagocytic activity. H37Rv multiplied most actively within WI-38 cells when compared to H37Ra and BCG Tokyo, suggesting that the ability to invade and survive within non-phagocytic cells reflects the more active invasion of virulent M. tuberculosis than avirulent M. tuberculosis. The assay system used in this study may help us to clarify the virulence of tubercle bacilli in vitro.