A method of labelling biologically active proteins with the alpha emmitting halogen211At is presented. The labelling procedure is discussed with reference to the chemistry of astatine. Proteins which have been
labelled retain approximately 50% of their original biological activity. Using cell specific labelled proteins, dose response
curves are given indicating that such reagents are extremely cytotoxic, D37 human CML cells=5 atmos211At/cell. The research potential of211At labelled biologically active proteins is briefly discussed.