Recent investigations concerning the significance of porphyrins in erythrocytes indicate a need for a rapid quantitative assay, easily adaptable to routine clinical use. To fill this need, we have developed a simplified procedure for the measurement of porphyrins (essentially protoporphyrin) in whole blood. Blood is stirred with acetone—ethyl acetate and the porphyrins are extracted with formic acid—ether. The porphyrins are then extracted into HCl and measured spectrophotometrically at three wavelengths, a procedure that allows a corrected absorbance to be calculated, to minimize error caused by interfering substances. The assay requires little apparatus, is simple and dependable, and the results correlate well with those obtained by use of a more complex conventional procedure.