Previous results, presented in abstract form, indicate that replacement of thromboplastin with a mixture of phospholipid and truncated soluble tissue factor apoprotein results in a coagulation assay that can directly measure plasma factor Vila levels without interference from zymogen factor VII (Atherosclerosis Thromb 11:1544a, 1991 [abstr]). We have exploited the specificity and sensitivity of such a factor Vila specific coagulation assay to directly assess the in vivo relationship of factor VIII and factor IX on the production of factor Vila levels under nonthrombotic and nonstimulatory conditions. Normal individuals (n = 20) were found to possess an average circulating factor Vila level corresponding to 4.34 ± 1.57 ng/mL, or approximately 1% of their total factor VII antigen. Severe factor VIII deficient patients (n = 13) possessed a slightly lower but statistically significant (P <. 01) decrease in their basal factor Vila levels (2.69 ± 1.52 ng/mL), corresponding to ≈60% of that observed in normal individuals. On the other hand, severe factor IX deficient patients (n = 7) were found to possess even lower levels of factor Vila corresponding to 0.33 ±0.15 ng/mL, or less than 10% of that observed in normal individuals. Measurement of total factor VII antigen levels shows that the variation in basal factor Vila levels stems from differences in the degree of factor VII activation as opposed to differences in factor VII antigen levels. Our present data are consistent with the hypothesis that factor IXa is the principal in vivo activator of factor VII under basal conditions.