The reduction of transmitted light at 650 nm caused by scattering from fibrin polymers as they develop in a clotting plasma sample has proven effective for following the course of the coagulation. An instrument designed on this principle and using entirely solid-state electronics has yielded clotting time reproducible to within ±15%, and has enabled discrimination between induction and fibrin polymerisation times. Data on vapour-deposited and sputtered films of polytetrafluorethylene, under study as thromboresistant surfaces, are presented to illustrate the uses of this coagulometer.