This multicentre study compares in parallel nine commercial coagulometers and manual tilt-tube methods for their capacity to distinguish two pairs of lyophilized plasmas with slightly different prothrombin times (PTs). The study was based on the rationale that the more precise a method is, the better it is able to discriminate between plasmas with similar PTs. Good or excellent discrimination, as judged by higher Student t values, were achieved slightly more frequently by coagulometers than by manual methods, particularly when paris of plasmas with smaller PT differences were tested.