A number of workers have examined protein C in relation to other vitamin K dependent factors during warfarin therapy and successfully identified protein C deficient patients by ratio calculation. However, protein S deficiency has not been addressed in this manner. This study compares protein C and protein S by functional and antigenic determination with procoagulant factors of similar half life (Factors VII and II) in an attempt to identify protein C and protein S deficient patients whilst on oral anticoagulant therapy. Procoagulant and anticoagulant factors were compared by linear regression in a population of normal blood donors and patients on stabilized warfarin therapy to obtain expected values for protein C and protein S dependent upon FVII and FII levels, respectively. Observed over expected values for protein C and protein S were calculated for individual patients and normal ranges derived. Comparison of similarly calculated observed over expected protein C and protein S ratios with these normal ranges successfully identified known protein C and protein S deficient patients who were taking warfarin at time of testing.