Restriction fragment length polymorphisms serve as valuable means for carrier detection of hereditary diseases. Factor VIII gene was recently cloned and sequenced, and an intragenic Bcl I polymorphism was found to be useful for carrier detection. We searched carriers of hemophilia A using a probe F8A which can detect the Bcl I polymorphism. Southern blots of Bcl I-digested DNA revealed polymorphic 1.2kb and 0.9kb bands by hybridization with the F8A probe. Of 15 unrelated women, 8 had both bands, 2 had a single 1.2kb band, and 5had a single 0.9 band. Study of four families in which carrier detection was possible by the Bcl I plymorphism analysis is presented in this paper: daughters of the three families were determined to be carrier and a daughter of the other family to be non-carrier. There is an inherent limitation in applying this method to carrier detection: the mothers must be heterozygous for the restriction fragment length polymorphism. However, use of multiple restriction fragment length polymorphisms will reduce the limitation.