Fujiwara trait, the first case of kininogen deficiency in Japan previously reported which did not show any clinical symptom except the prolonged activated partial thromboplastin time was further examined. The activated partial thromboplastin time of the patient was corrected by addition of normal, Factor XII deficient or Fletcher plasma, but not corrected by Fitzgerald or Williams plasma. It was also corrected by addition of highly purified bovine or human high molecular weight (HMW) kininogen, but not by low molecular weight (LMW) kininogen. When total kininogen was measured as the amount of bradykinin released by trypsin, only a trace amount was detected in Fujiwara as well as Williams plasma. No immunoreactive protein against anti-human-HMW-kininogen nor anti-human-LMW-kininogen was found in Fujiwara plasma. Acetone-kaolin-activated plasma kallikrein was not generated by Fujiwara plasma. Substitution with normal plasma in various ratios showed the generation of various plasma kallikrein activities. Calculations with these activities of mixed plasma gave the prekallikrein content of Fujiwara trait plasma about 30% of the normal level. These results suggest that Fujiwara trait is very similar to Williams trait in that both plasmas were deficient in HMW and LMW kininogens with reduced content of prekallikrein.