In 1946, Callender et al. first reported C"' antigen and the corresponding antibody in Rh system. We found one case of anti-C", which is very rare in Japanese, although not so rare among white people, in a 69-year-old male patient with carcinoma of the bile duct. The patient had been transfused many times during the three months before anti-C' was detected, but never prior to that. The titers of the antibody in saline method were 128, 16 and 16 at 37oC, 20oC, and 4oC respectively, 2, 000 by

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... min method, and moreover 256 by bromelin method. It was not detectable by indirect antiglobulin test. This antibody was caused by unknown origin. Because C' antigen is extremely rare in Japanese, so it is easy to obtain compatible blood for a patient with the anti-C'.