We report genetic abnormalities of protein C gene in a male infant who developed neonatal purpura fulminans. DNA-sequence analysis of all exons in protein C gene in this family revealed two mutations. The first abnormality, derived from the mother, was a deletion of one of four consecutive G at nucleotide number 10758 in exon IX which would result in a frame shift mutation and completely change amino acid sequence from Gly381 in the carboxyl-terminal region of protein C. The second abnormality, derived from the father, was a single nucleotide mutation from G to A in the codon (GAG to AAG) at nucleotide number 2977 in exon III, which would result in a substitution of Lys for gamma-carboxyglutamic acid (Gla)26. This change would be responsible for the reduced immunological protein C levels of the patient and the father, estimated by a monoclonal antibody which recognizes the Gla-domain in a Ca(2+)-dependent manner (3.8% and 57%, respectively). Partially purified abnormal protein C from the father's plasma showed a normal amidolytic activity and a change in the electrophoretic mobility. We detected the above mutations in his family members using two methods; one was a creation of new restriction enzyme sites using mutagenic primers and the other was single nucleotide primer extension. Both methods are rapid and useful for the diagnosis of prenatal protein C abnormalities.