A simple, rapid method for the determination of cholesterol in plasma and tissue using o-phthalaldehyde is presented. Comparison of this method with the FeCl(3) method gave identical results. However, the o-phthalaldehyde determination is three times more sensitive than the FeCl(3) determination (molar extinction coefficients of 11,610 and 33,440 for FeCl(3) and o-phthalaldehyde, respectively), it takes less time to complete, and the color developed is more stable. The o-phthalaldehyde method can be used to assay free and esterified cholesterol directly after thin-layer chromatographic separation.