To determine the specificity and sensitivity of current technics for detecting carriers of classic hemophilia, three investigators simultaneously tested obligate carriers and noncarriers for immunologic and procoagulant factor VIII activity. Overall correct classification ranged from 72 per cent (36 of 50) to 94 per cent (47 of 50). The maximum accuracy obtained with the same linear-discriminant-function method on all data was 90 per cent (26 of 29) in detecting carriers without misclassifying normal persons as carriers (none of 21). Lowest accuracy by the same technic was 66 per cent (19 of 29) carrier detection while misclassifying 19 per cent (four of 21) normal persons. Precision of testing for both factor VII activity and antigen was high (standard deviations from 0.004 to 0.026 on a log scale). Differences between participants seemed related to laboratory technics rather than to statistical methods. The factor VIII activity/antigen measurement is a valid technic for detecting in the carrier state of hemophilia A.