Medical Sciences. In the article “Gadolinium(III) texaphyrin: A tumor selective radiation sensitizer that is detectable by MRI” by Stuart W. Young, Fan Qing, Anthony Harriman, Jonathan L. Sessler, William C. Dow, Tarak D. Mody, Gregory W. Hemmi, Yunpeng Hao, and Richard A. Miller, which appeared in number 13, June 25, 1996, of Proc. Natl. Acad. Sci. USA (93, 6610–6615), the following correction should be noted. It has come to our attention that the radiation sensitivity of the HT29 control cell line reported in Fig. 2 on page 6611 is inconsistent with that reported in the cited literature (33). Consequently, the in vitro HT29 radiation sensitization experiments have been repeated with Gd-tex²⁺ (compound 1). Radiation enhancement comparable to our original findings was observed at doses between 8 and 20 Gy. The results indicate that the Gy scale reported on the x-axis of Fig. 2 is incorrect. We apologize for this error. The conclusions reached in the article remain unchanged.