Summary. A ‘blind’ study has been made to try to find out if it is possible to diagnose carriers of haemophilia. A group of 34 obligatory carriers of haemophilia were compared with 34 normal women. Levels of factor VIII activity, factor VIII-related antigen, factor V and ratio of factor VIII activity to factor VIII-related antigen were measured. In the carrier group the mean level of factor VIII activity and the mean level of the ratio of activity to antigen were each approximately half of those found in the normal women. The mean level of factor V was the same in both groups of women. By setting the lower limit of normal at the lowest level of the different factors found in the normal women, 12 out of 34 (35%) carriers could be distinguished on the basis of their factor VIII level alone; 24 out of 34 (71%) could be detected on the basis of the ratio of factor VIII activity to factor VIII related antigen and 25 out of 34 (73%) could be detected if both factor VIII activity and the ratio were taken into account. It is concluded that consideration of both the level of factor VIII activity and the ratio of factor VIII activity to factor VIII-related antigen is of some value in detecting carriers of haemophilia. The number of carriers detected (73%) in the present study is not as high as that found by other workers.