Hereditary motor and sensory neuropathy-Ib (HMSN-Ib) is a common autosomal dominant disorder linked to the Duffy blood group locus on human chromosome 1. The gene for antithrombin III (AT3) is also located on the long arm of chromosome 1. Using a DNA restriction fragment length polymorphism for AT3, we have investigated the genetic linkage relationship of all three markers (HMSN-Ib, Duffy, and AT3) in two affected families. Neither HMSN-Ib nor Duffy was tightly linked to AT3. The loci for both HMSN-Ib and Duffy must be close to the centromere on chromosome 1, but precise localization and gene order require study of additional markers and more families.