IN a recent study1 the strength of the bond between Portland cement paste and sawn aggregate surfaces was found to be less than that of the paste matrix. In some instances therefore, for example in the case of the flexural strength of plain concrete containing smooth aggregate, cement-aggregate bond is likely to be the factor controlling strength. Although several aspects of the mode of failure of concrete could be explained by this concept, it was soon clear that a number of others which should also have been explained were, in fact, anomalous. The form of the departures from expected behaviour suggested that cement-aggregate bond-strength might depend on aggregate size, and the following experiments were therefore performed to check this possibility.