Prothrombin was purified from normal blood donors and individuals heterozygous for prothrombin type 3. Comparison of the purified prothrombin preparations by tryptic peptide mapping, amino acid analysis and automated sequencing after thrombin digestion, has indicated that prothrombin type 3 results from the substitution of a lysine residue for glutamic acid at position 157. This substitution can result from a single base change in the structural gene and explains the relatively slow electrophoretic mobility of prothrombin type 3 at alkaline pH. An additional thrombin cleavage site in profragment 1 has been identified at arginine 54 by automated sequence analysis of thrombin digests by prothrombin.