Stubel in 1914 (1) first noted with an ultramicroscope that the fibrin strands produced from hemophilic blood are thicker than from normal blood. He attributed the thicker strands to the slower rate of clotting in hemophilic blood. Later in the same year Howell (2) performing similar kinds of experiments reported an experiment in which cat's plasma was clotted with thrombin: "A large amount of thrombin caused very rapid clotting with the production of small but distinct needles, while with a small amount of thrombin the clotting was slow and the needles were relatively large." In 1947, Ferry and Morrison (3) reported a similar microscopic observation in which decreasing thrombin concentration produced larger fibrin strands. They observed this at pH 6.0 since at pH 7.0 the strands were too fine to be seen. Morrison and Scudder (4) working at pH 6.4 believed there was no change in the thickness of the fibrin strands with varying thrombin concentrations. This was based on electron microscope photographs. A light scattering method for the determination of the average fibrin strand width has been reported previously from this laboratory (5). With this technique it is now possible to examine the effect of thrombin concentration on fibrin strand width in more detail.Although numerous papers have been written on the relation of platelets to clot retraction 8 (7,