Eperiments were carried out to measure the shielding effectiveness of concrete slabs, both ribbed and equivalent smeared configurations, for broad parallel beams of essentially monoenergetic radiation. The broad-beam source of cobalt-60 radiation collimated to a thin beam, and by use of a line detector. Angels of incidence of 0 C (normal), 45 C oand 60 C were investigated. Results were compared with previos Monte Carlo calvulations and shown to be consistent with them, provided certain factors, such as the angular spread of the collimated beam, were taken into account.