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Growing interest is observed in chromogenic substrate assays, because of their better precision, performance and possibilities for automation. Applied to a Cobas Bio centrifugal analyzer, we compared Nycotest-Chrom (N-test) and Thromboquant-PT (Tbq) with Thrombotest (TT). Precision tests were performed with samples from 4 plasma pools: normal (45 sec), low (100 sec), middle (150 sec) and high (200 sec) segments of the therapeutic range of TT. N-test had the best precision profile in both intraassay and interassay determinations compared with Tbq. Both chromogenic substrate assays were better than TT in this respect. By orthogonal regression a provisional therapeutic range was calculated from 312 determinations and later adjusted in a confirmation experiment with natural logarithm regression in 946 determinations. Compared with a TT range of 105–180 sec, the therapeutic ranges were 71–120 sec for N-test and 63–103 sec for Tbq. In a clinical therapeutic control phase, 110 patients were randomized in equal proportions to two groups A and B. Every 2 weeks for a period of 16 weeks, blood samples were tested for N-test, Tbq and TT. In group A, dose adjustment was based on N-test, and in group B on Tbq. The monitoring physician was blinded for Tbq and TT in group A and for N-test and TT in group B. No differences were found between the groups for mean TT, N-test, Tbq or mean dosage, nor differences were found in complication rates. A definite therapeutic range was compiled using sensitivity/specificity curves together with their 95% confidence limits. Given TT 105–180 sec, the therapeutic range of N-test is from 80 (95% confidence limits: 77–82) to 110 (95% confidence limits: 108–115) sec, and of Tbq from 68 (95% confidence limits: 66–71) to 95 (95% confidence limits: 91–98) sec. It was concluded that the two chromogenic substrate assays performed equally safe in the monitoring of patients on oral anticoagulant therapy, despite differences in diagnostic correspondence with the reference TT.