Polyacrylamide gel isoelectric focusing (PAGIEF) of neuraminidase treated EDTA plasma samples at pH 7-9 with 1.0 M urea followed by semi-dry horizontal electroblotting with enzyme immunoassay was done for the detection of factor I (IF) allotypes in 604 unrelated Japanese blood donors living in Tokyo. In these samples three common types, IF A, IF AB, and IF B, and a new variant were observed. A new variant which had major bands in a more acidic region than IF A was designated IF A1. Family data was in accordance with the Mendelian inheritance. The allele frequencies were estimated as IF*B = 0.8924, IF*A = 0.1068 and IF*A1 = 0.0008, respectively. The IF protein level of various allotypes was measured using the single radial immunodiffusion technique. The mean IF protein level for IF B was slightly higher than that for IF A (P less than 0.001) and that for IF A1B (P less than 0.05).