Highly purified single-chain factor VII was isolated from plasma and used to generate monospecific antibodies. A double-antibody equilibrium radioimmunoassay was constructed. The assay was tested for and met all the criteria required for a specific, sensitive, and accurate determination of factor VII in plasma. The range of sensitivity of the assay was between 1 and 500 ng factor VII/ml, and the coefficient of variation was 1%-3% within assay and 12%-16% between assays. Pure factor VII and plasmic factor VII from normal, warfarin-treated, and hereditary deficient individuals inhibited competition assays with parallel slopes, indicating the expression of similar epitopes by these molecules and validating the measurement of this protein in plasma. The concentration of factor VII in normal plasma (n = 41) was 470 +/- 112 ng/ml, and the measurement of factor VII antigen correlated with activity (r = 0.82). Factor VII concentration in the plasma of individuals on warfarin therapy (n = 24) was 238 +/- 73 ng/ml. Factor VII activity was about 38% of normal and correlated less well with factor VII antigen (r = 0.53). The specific activity of these molecules was 78% of normal (p less than 0.01), suggesting the presence of nonfunctional or partially functional molecules in the circulation of individuals undergoing drug therapy. Analysis of two hereditary deficient patients revealed that, while there were significant levels of factor VII protein, the procoagulant activity was less than 2%, indicating a discordant relationship of these parameters in individuals expressing the deficient factor VII phenotype.