Up to the present time it has been possible to obtain two different figures for the bitterness of a beer according to whether the analysis has been made and calculated by the Rigby & Bethune Second Method or by the Moltke & Meilgaard and related methods. By co-ordinating the results obtained by the Isohumulone Sub-committee of the A.S.B.C. with those of the Analysis Committee of the E.B.C., it has been shown that the results by the two types of method are related in such a way that at 28 parts per million they agree; but, progressively, above this value the Moltke & Meilgaard figures become larger than the Rigby & Bethune ones for the same beer. The opposite is true below 28 p.p.m. Since, nevertheless, a good relationship exists between the results by the two types of method, they could all be stated on a uniform scale. There are arguments in favour of adopting the Rigby & Bethune calculation, and the Societies have agreed to adopt it and to name the results “International Bitterness Units” (in America “Isohumulone Bitterness Units”). As a result, inter-continentally agreed Tables are published here for converting Moltke & Meilgaard results (or for converting the ultraviolet measurements directly) into I.B.U.s. For the great majority of beers—those made with fresh or well-stored hops—the results by the two types of method agree, if they have been calculated in this way. However, the E.B.C. Analysis Committee has studied also beers made from old or deteriorated hops and here, even when calculated in the new units, the two types of method give discrepant results. It has been shown by chromatography that the Rigby & Bethune washing of the iso-octane extract with acid methanol removes from such old-hop beers another group of compounds—so leaving purified isohumulones. However, the compounds removed are also bitter, so that the value of the Moltke & Meilgaard method for such beers is explained. It seems probably that the extra bittering compounds, additional to isohumulones in beers made from deteriorated hops, are water-soluble derivatives of the resin acids.