The effect of a cytotoxin isolated from Shigella shigae has been tested on different cell lines. HeLa S3 cells, as well as some other human carcinoma cells, were killed by picomolar to femtomolar concentrations of the pure toxin, whereas certain other human carcinoma cells and a variety of non-epithelial cells from human tissue and from various animal tissues were resistant to-nanomolar concentrations of the toxin. Binding studies with 125 I-labelled Shigella shigae cytotoxin showed that the sensitive HeLa S3 cells contain 1.3 x 10(6) binding sites per cell, whereas in an insensitive HeLa cell line 2.6 x 10(5) sites per cell were measured. In all cases the apparent association constant, Ká was found to be about 10(10) M-1. The binding occurred fairly rapidly, whereas dissociation of bound toxin occurred at a very slow rate, even in the presence of excess unlabaled toxin. All toxin sensitive cell lines bound similar amounts of toxin as HeLa S3 cells, whereas some of the resistant cell lines did not contain measurable amounts of toxin receptors.