Methods based on turbidity and permeability, for measurement of mass-length ratio of fibrin fibres developed in pure fibrinogen solution, have been evaluated in respect of their applicability to human plasma. Theoretical assumptions made in the calculation of mass-length ratios in plasma have been critically examined. Methods of handling plasma, reproducibility of technique and the influence of age and sex have been investigated. The anticoagulant used, as well as other factors such as time, venepuncture and effects of calcium and fibrinolytic inhibitors are fully explored. With suitable standardization the methods are acceptable for application to clinical studies and are reproducible.