We report the characterization of the genetic defect in a family with hereditary type-II protein C (PC) deficiency. The propositus is a 28-year-old woman with a history of thrombosis. Her PC activity level (58%) and PC antigen level (115%) are compatible with the diagnosis of type-II PC deficiency. Her asymptomatic sister is also PC deficient. Analysis of the PC gene of the propositus revealed a point mutation (G to A) at nucleotide 8856, which results in the replacement of Gly381 by Ser in the heavy chain of PC. The amino acid change occurs close to the active-site serine at a residue which is highly conserved among the serine proteases. The mutation is also present in the PC gene of the propositus' sister. Her brother, who is asymptomatic, has a normal genotype with respect to the mutation at nucleotide 8856.