A family with a new factor X defect is reported. The proposita is a 56-year-old female. She is asymptomatic and no consanguinity is present between the parents. The main features of the defect are: prolongation of prothrombin time and derivative tests but normal partial thromboplastin time. Factor X was found to be low (about 25-30% of normal) only if tissue thromboplastins were used in the assay system. Chromogenic substrate S-2222 also yielded decreased factor X levels. However, factor X activity was normal with cephalin and cephalin-RVV mixture. Factor X antigen was normal in three immunological systems (electroimmunoassay, an Elisa method and laser nephelometry). Crossed immunoelectrophoresis and antigen-antibody kinetics recorded in a laser nephelometer failed to show major differences from normal factor X. Both sons of the proposita, the father and other family members showed slightly decreased factor X levels and normal factor X antigen and were considered heterozygous for the abnormality. The toponym factor X Padua is proposed to indicate this peculiar abnormality.