Irinotecan inhibits the in vitro growth of AT/RT cells. Triplicate cultures of AT/RT cell lines were cultured in the presence of increasing concentrations of irinotecan. Following four days in culture, cell growth inhibition was measured by Alamar blue assay. Percentage of cell survival was calculated by comparison of treated cells to control cells. Similar results were obtained in five separate experiments. IC50 values obtained from these data are given in Table 2.