The sedimentation method for measuring fluorescent microspheres (FM) to determine organ blood flow has been modified to improve measurement in brain tissue. The addition of Triton X-100 as a solubilizer has been developed to reduce clumping of the centrifuged pellet and thereby increase FM yield from brain tissue samples. In addition , one color is reserved to add as an internal standard in order to determine if the yield of microspheres is adequate for each sample. Polypropylene tubes are used in the extraction which includes a 48 hour digestion of the tissue in 2 M ethanolic KOH and 0.5% Tween 80. After centrifugation and the removal of most of the supernatant, a 1% solution of Triton X-100 is used to resuspend the pellet. Triton X-100 is an improvement over the Tween 80 used at this point by others as clumping of the pellet after centrifugation is greatly reduced, and resuspension of the pellet in the next wash step is thereby facilitated. The Triton X-100 also eliminates the problem of a second lipid layer on the surface of the aqueous wash steps. This assay provides a suitable method for determining the number of FM in brain and other tissues containing a large fraction of lipids.