SummaryIn an attempt to discover a laboratory test by means of which female carriers of the gene For the Duchenne type muscular dystrophy could be identified, creatine and creatinine excretion has been studied in forty‐eight female relatives of patients suffering from this variety of muscular iystrophy. The activity in the serum of aldolase, of serum glutamic oxalacetic transaminase (SGOT) and of serum glutamic pyruvic transaminase (SGPT) has also been determined in thirty‐six female relatives. The females tested were divided into three groups:(a)known carriers,(b)possible carriers, and(c)probable non‐carriers. No significant differences were discovered between the results obtained in each of these three groups. No method of identifying female carriers has yet been discovered.We are grateful to Dr A. L. Latner for his advice and for providing facilities in the Department of Chemical Pathology of the Royal Victoria Infirmary for biochemical estimations to be carried out in the Newcastle upon Tyne series. Creatine and creatinine estimations on the female relatives of patients living in the London area were carried out in the Department of Chemical Pathology, the Institute of Neurology, the National Hospital, Queen Square, by kind permission of Prof. J. N. Cumings. The pedigree was prepared by Miss M. Mustart of the Department of Photography, King's College, Newcastle upon Tyne, and statistical analyses were performed by Mrs D. Weightman of the Department of Industrial Health, King's Co