Limited information is available regarding molecular abnormalities associated with derived factor X deficiencies. In order to assess the types of molecular aberrations that may occur in this group of haemostatic diseases we have analysed plasmas of 33 individuals from 28 kindred exhibiting factor X deficiency. These included those of hereditary type, transiently acquired deficiency as well as factor X deficiency associated with amyloidosis. Plasmas were analysed by one-stage assays for factor X activation by the extrinsic coagulation pathway, intrinsic coagulation pathway and Russell's viper venom. Selected plasmas were analysed in two-stage assays. Normal factor X concentration by specific radioimmunoassay was 6.38 +/- 1.29 micrograms/ml. Factor X associated with factor X deficiency were grouped by their specific activities measured for the three activation pathways and antigen concentration. The results suggest that a broad spectrum of molecular aberrations exist in the factor X deficiency states. The most common group of factor X deficiency was associated with abnormal activation of factor X by all three pathways. Variants of factor X associated with primary amyloidosis and transient acquired deficiency appeared to be abnormal molecules and not just reduced factor X concentration. Hereditary abnormal factor X molecules include the spectrum of potential defective molecules. The relationship of factor X structure to function and the heterogeneity of these defective molecules is discussed.