Variant factor X in an individual with a mild bleeding tendency was suspected based on deficient procoagulant activity (10-20% of normal) and antigen (30-35% of normal) levels of plasma factor X. Heteroduplex analysis of factor X gene exons indicated heterozygosity for mutations in both exons 6 and 4, confirmed by direct sequencing of the amplified exons. Substitution of C by T at nucleotide position 13,984 (Arg-139 to Cys) was found in the factor X gene exon 6 of the propositus. This mutation creates a BsmI site and the patient tested heterozygous for the BsmI cleavage involved, as did one of his two daughters. In addition, exon 4 was found to have the normal A and a novel C (Asn-57 to Thr) at nucleotide position 9338. The exon 4 mutation creates a BsaJI site, detectable after amplification mismatch to remove an existing BsaJI site. Both the patient and the second of his two daughters were heterozygous for this cleavage. The two variant proteins are called factors XWenatchee I (Arg-139 to Cys) and II (Asn-57 to Thr). A mixed variant isolate derived from the plasma of the propositus exhibited heavy/light chains of normal size, as well as an apparent single-chain molecule not dissociable by reducing agent. A single-chain molecule would be predicted for form I, if the mutation blocks processing cleavages that normally remove a tripeptide interposed between the heavy and light chains. A Western blot of partially purified factor X from the daughter who inherited the form I defect revealed a component migrating the same as the putative single-chain species. Based upon the factor X activity vs. antigen ratios for the propositus and both daughters, both forms I and II are probably dysfunctional molecules.