An enzymatic method for the determination of the concentration of free heme. A, The evolution of luminescence intensity emitted by the reactions in Eq. (1a,b,c) at five concentrations of heme and in its absence. B, Illustration of the determination of the concentration of free heme in the dialysate. The standard curve, shown in solid symbols, relates known concentration of heme to the rate constant k of decay of the luminescent intensity determined from evolution curves similar to those shown in A. The constant k is determined for six samples of dialysate. The unknown concentration of free heme in the dialysate is determined from the average value of k.