In analysed systems representing artificially reconstituted plasmas of known composition with respect to prothrombin. (pro)convertin, and AcG. One-stage clotting-times with Ca-thromboplastin, essentially in the manner of the “prothrombin time” (Quick) test, show: (1) prothrombin concentration is a major variable. (2) (pro) convertin concentration is a minor variable, which, except at very low(10) ... [Show full abstract] levels, affects the clotting-times significantly only at very low prothrombin levels. (3) High (pro)convertin levels, short of an unexplained inhibitory phenomenon (minor), do not compensate significantly for low levels of prothrombin or of AcG. (4) AcG levels are a major variable. (5) Excessively high (200-300% of'normal') AcG levels can compensate for moderately low prothrombin and proconvertin, causing the one-stage test to lose significance as a measure of these other two factors. (6) It seems valid to apply this last result to explain the normal “prothrombin time” (and lack of bleeding) in normal healthy newborn infants(1).