Growth-inhibitory effects of a partially purified bacteriocin derived from Mycobacterium smegmatis ATCC 14468 on various animal cells transformed by tumor viruses, human malignant cells, and normal cells in the same species were studied. A growth-inhibitory effect of the bacteriocin on these cultured cells was determined by counting the residual cells. The bacteriocin inhibited virally transformed animal cells (mKS-A TU-7, 155-4 T2, and XC cells) and human malignant cells (AS-II and HGC-27 cells). The inhibitory effect increased with an increase in the bacteriocin activity. The bacteriocin sensitivities of transformed animal cells were relatively higher than were those of human malignant cells, while normal cells in the same species were practically insensitive to the bacteriocin. Differences in the degree of bacteriocin sensitivity were observed among tumor cell lines. Simian virus (SV) 40-transformed hamster cells (TSV-5 cells), which grow rapidly, were less sensitive to the bacteriocin. The cell membrane of SV40-transformed BALB/c mouse cells (mKS-A TU-7 cells) adsorbed the bacteriocin much more than did the cell membrane of nontransformed BALB/3T3 cells. The results seem to indicate that the inhibitory effect of bacteriocin 14468 on cultured mammalian cells probably depends on the binding sites for the bacteriocin which appear or increase by malignant transformation on cytoplasmic membrane.