An 18-month-old child, who had no evidence of liver disease, malabsorption, or chronic ingestion of coumarin compounds, was found to have plasma deficiencies of factors II, VII, IX and X. Assays for factor II and X by immunological techniques (antibody neutralization and immunoelectrophoresis) revealed normal or elevated antigenic activity of these factors, suggesting the presence of abnormal protein variants in the patient's plasma. On two-dimensional immunoelectrophoresis of the patient's plasma in calcium, a normal and an abnormal population of prothrombin were seen. The abnormal prothrombin had a mobility more anodal than that of normal prothrombin, but less anodal than that of acarboxyprothrombin. The abnormal prothrombin, in contrast to acarboxyprothrombin, adsorbed readily to both aluminum hydroxide and barium citrate, and could be identified by two-dimensional immunoelectrophoresis of a barium citrate eluate. We suspect that the abnormal variant represents a partially carboxylated prothrombin.