The Biocolor Fastin™ Elastin Assay is a quantitative colorimetric dye-binding method for the analysis of elastins released into tissue culture medium and extracted from biological materials. The dye label employed is 5,10,15,20-tetraphenyl-21H,23H-porphine tetra-sulfonate (TPPS). Elastin forms that can be measured as α-elastin: soluble tropoelastins, lathyrogenic elastins, insoluble elastins (following solubilization to elastin polypeptides [α-elastin, κ-elastin]). In vitro application: soluble elastin secreted into cell culture medium may be measured directly. Samples should be free from particulate material. Tropoelastin is the elastin form usually found in cell-culture medium during in vitro cell culture. In vivo application: an extraction method of insoluble elastin from tissue and conversion to soluble α-elastin or soluble κ-elastin is given. After solubilization the samples are assayed by the procedure described for soluble elastin. Sensitivity: 5 µg. Assay Time: 4 hours.