The importance of the haemagglutination-inhibition test does not need to be stressed. It is a fact that the inhibition titre of a single serum has no constant value, but may vary in repeated estimations. This may be due to the use of different virus or erythrocyte suspensions or to differences in environmental circumstances, such as temperature. Duplicate tests carried out at the same time and with the same materials yield identical results. This makes it necessary to compare a serum obtained during convalescence from the same case. The ratio between the two titres shows whether an influenza infection has taken place or not. Although it is commonly accepted that a titre rising by two twofold dilutions (a fourfold rise) is significant, some workers will accept even a twofold rise (Sartwill & Long, 1948; Rasmussen, Stoles & Smadel, 1948). Such observations have been made on pairs of sera taken from the same indicidual with not more than a 5-week interval between them.