Elastic and viscous properties of fibrin clots were measured in the cone-plate test section of a Weissenberg Rheogonimeter. The tests applied included forced oscillations, free vibrations, and stress relaxation, all in shear. Two classes of clots were studied: (i) ligated clots, formed with the presence of the enzyme fibrinoligase, and (ii) non-ligated clots, formed in the absence of fibrinoligase. The clots, formed in a cone-plate test section, gradually became stiffer over a period of about 1 hr, thereafter being stable for 2 additional hr. The present study deals with quantities measured during this stable period. Compared to the non-ligated clots, the ligated clots, representing physiologic conditions, were found to be stiffer and less viscous. Furthermore, the former clots became even less stiff when they were cyclically strained for 30 min; the loss of rigidity, however, was mostly recoverable if they were left undisturbed subsequently for 40–60 min. For the strain range tested (up to 20 per cent), stress-strain curves for all clots were nonlinear, the main non-linearities being at small strains. The tangent modulus at large strains was constant for most clots. The storage modulus of all clots is strongly dependent on the fibrinogen concentration and on the total ionic strength of the clotting mixtures: The stiffest ligated clots are obtained at an ionic strength of 0.17. Less rigid clots are produced at smaller and greater ionic strengths. Non-ligated clots, tested at 0.03 Hz, dissipated 20–30 per cent of the input energy, whereas the ligated clots dissipated less than 5 per cent. Both ligated and non-ligated clots are more rigid if test frequencies above 1 Hz are applied. Below 1 Hz, the storage modulus is fairly constant. Clots cyclically strained during the formation period seem to be only one-fourth as rigid as clots formed undisturbed.