An abnormal plasminogen was discovered because of a decreased level of plasminogen activity in plasma contrasting with a normal level of plasminogen antigen concentration. The same discrepancy was found in the purified plasminogen. The molecular abnormality seems to be inherited. The patient is a heterozygote. The experimental findings can be explained by assuming that half of the plasminogen is normal, while the other half is an inactive mutant protein, without catalytic activity after SK or UK addition. There was no binding of labeled DFP and a decreased binding of TLCK to the abnormal plasminogen. The role of the abnormal plasminogen in thrombotic tendency is uncertain since the patient is the only one who has suffered a thrombotic accident, while her relatives who present the same defective plasminogen have not had thrombotic problems.