A comparison has been made between the prothrombin time test using British Comparative Thromboplastin (BCT) and a chromogenic substrate assay for factor VII in the assessment of laboratory control of oral anticoagulants in short-term and long-term patients. Opportunity was also taken to compare the findings with parallel results obtained with the venous Thrombotest technique and a specific clotting assay for factor VII. There was good agreement between the amidolytic factor VII assay, using a method modified from Seligsohn et al (1978) with the Quick test using BCT and Thrombotest in 60 long-term patients. Tests in 53 patients within the first 3 weeks of starting oral anticoagulant administration gave less satisfactory agreement between the above amidolytic method and the conventional tests. In contrast, there was a good correlation between the two conventional tests in both groups and also between the clotting and amidolytic factor VII method. Although the results are an improvement on previous, less satisfactory correlations between the BCT prothrombin time method and amidolytic assays for factor II and X, the present study indicates the limitations of a specific clotting assay versus a broad spectrum extrinsic clotting test in oral anticoagulant control. While not warranting the routine use of the chromogenic assay for factor VII in place of the prothrombin time using BCT, the factor VII amidolytic assay offers a limited but dependable guide to dosage in long-term patients. The complexity of the technique in its present form militates against its adoption for routine anticoagulant control in hospital laboratories.