Cytotoxicity and adsorption of pyocin S2 produced by Pseudomonas aeruginosa M47 (PAO 3047) to virally transformed mammalian cells, human malignant cells and normal cells in the same species were studied. Pyocin S2 inhibited the growth of not only tumor cells (XC, TSV-5, mKS-A TU-7, HeLa-S3 and AS-II cells) but also normal cells (BALB/3T3 and BHK 21 cells). The inhibitory effects on the cells increased with an increase of pyocin S2 activity. On the other hand, there were some tumor cells (155-4 T2 and HCG-27 cells) and normal cells (normal rat kidney and human embryo lung cells) which were resistant to pyocin S2. The pyocin S2 activity was neutralized by the cell membrane preparations from pyocin S2-sensitive cells, but not by those from pyocin-resistant cells. This neutralization ability was inhibited by high concentrations of D-galactose, N-acetyl-D-galactosamine and N-acetyl neuraminic acid and completely destroyed by periodate and neuraminidase. The inhibition by the saccharides was concentration dependent. These results suggest that the toxicity of pyocin S2 in the cell membrane and further, that the carbohydrate moiety, especially of D-galactose, N-acetyl-D-galactosamine and sialic acid, may play an important role as an initial binding site for pyocin S2.