Eight patients with severe or moderately severe haemophilia B were treated for six months, according to three schemata for two months each: 18, twice 18, or twice 9 U of factor IX per kg body-weight weekly. The sequence of the six possible treatment schemes was determined strictly at random. One patient had to be excluded because he developed hepatitis, another because allergic signs developed. In the pre-trial period the number of bleedings per two months had been about 40, as recorded by the patients. The bleedings were reduced to nine after 18 U of factor IX per kg body-weight weekly, falling to two after twice 9 and twice 18 U/kg weekly. The patients themselves considered twice 9 U/kg as the ideal dosage. It is recommended that this dosage scheme should be used initially if one decides to employ permanent substitution in haemophilia B. Once freedom from bleedings and strengthening of the motor system have been achieved one can then try 18 U per kg body-weight once a week.