A factor B subtype, BF*Fb1, was first detected in Japanese by using isoelectric focusing or agarose gel electrophoresis in Tris/glycine/Veronal buffer. Our previous studies suggested that BF*Fb1 may be characteristic of some of Mongoloid populations. To investigate further distribution of BF*Fb1, samples randomly collected from Japanese in Yonaguni island of Japan and Cambodian were tested. BF*Fb1 was not observed in a Cambodian population whereas in a Japanese population of Yonaguni island, BF*Fb1 occurred at a frequency five times as high as those in main islands of Japan. In paternity cases, a Korean family with three offsprings was shown to transmit BF*Fb1 from the accused man to one of the offsprings. These data strongly indicate that BF*Fb1 is a marker gene for some of Mongoloid populations.