Treibs applies spectrophotometric measurement of absorption coefficients to the determination of hemin and coproporphyrin in yeast cultures. Vierordt''s principle for spectrophotometric analysis is applied, assuming that both substances follow Beer''s law. Upon the further assumption that the absorption coefficient of the colored substance is the same in two spectral regions, the following equation is derived: C=(e[long dash]e[image])/k, where C is the unknown concentration of one colored substance, e and e[image] are the absorption coefficients of the unknown compound in two spectral regions, k is a constant belonging to the known pigment. These assumptions are found to hold fairly well for yeast extracts. The technic of the measurement is described and the necessary changes in apparatus and precautions are discussed. Measurements of pure hemin in specially purified pyridin did not follow Beer''s law. It undergoes a change to hemochromogen which can be followed with the spectroscope. The transformation of hemin into hemochromogen takes place readily in the presence of a little hydrazine, but a large excess must be avoided as it renders the hemochromogen unstable. Measurements of the absorption spectrum of hemochromogen show that it obeys Beer''s law, as does coproporphyrin in pyridine solution, hence the Vierordt principle can be applied to hemin-coproporphyrin determinations if the hemin is first changed to hemochromogen. The accuracy of the coproporphyrin determination is only } that of the hemin, owing to the position of the bands in the spectral field and to the nature of the contaminants. Complex coproporphyrin salts do not increase the accuracy of the determination. Applied to yeast cultures the method may be accurate within about 10%, depending upon the skill of the observer. The observed values for certain cultures are presented.