A 2 2/12 years old boy developed a "lupus anticoagulant" after measles. Fibrinogen concentration and activity of factors II, V, VII, VIII, IX, X, XI and XII were normal, but partial thromboplastin time and thromboplastin time were prolonged. Thrombin time was within the normal range. Mixing normal plasma with the patient's plasma immediately prolonged the partial thromboplastin time, and incubation of the sample did not increase the inhibition. The immunglobulin fraction prepared from the patient's plasma on DEAE-sephacel also prolonged the partial thromboplastin time in cross mixing experiments. The thrombocyte count and thrombocyte functions were normal. In spite of the pathological partial thromboplastin and thromboplastin times, surgery was performed without excessive bleeding during the operation and without delayed wound healing.