ICRP Publication 21 contained data for protection against ionizing radiation from external sources. The data were of two kinds, one on the relationships between various radiation quantities, the other on the shielding properties of various materials. Some revised shielding data are now in ICRP Publication 33, which deals with external sources used in medicine: the other kind of data is considered here, but is not intended to apply to the irradiation of patients. The main reason for this revision is to adapt the data and the underlying approach to the Recommendations of the International Commission on Radiological Protection in ICRP Publication 26 and later relevant modifications. It is also necessary to take account of the report on radiation quantities and units from the International Commission on Radiation Units and Measurements and a subsequent report on the determination of dose equivalents. The third reason is to improve the original publication be amending or replacing some data.