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This study has demonstrated the feasibility of determining erythrocyte protoporphyrin concentration by the direct fluorimetric measurement of diluted whole blood. Excitation and emission wavelengths can be chosen to minimize heme (or hemoglobin) interference. Although results obtained by this method and an extraction method differed, the discrepancies were approximately constant and largely explainable on a basis of fluorescence quenching by heme. Nevertheless, several kinds of pathologic blood specimens having elevated protoporphyrin concentrations were easily detectable. Perhaps more important from a diagnostic sense, the method can be used to quantify directly the [PROTO]/[heme] ratio. This new approach to blood porphyrin assay should find extensive application in such public health problems as lead intoxication and iron deficiency.

The absorption spectrum of single and oriented crystals of iodine has been investigated in the wavelength region 7000–3000 A. Four strongly polarized bands and one weakly polarized absorption band have been observed, all of which are polarized in the ac plane of the crystal. All of the absorption bands are approximately 1000 cm−1 wide. The crystal‐band theory for iodine given by Rosenberg has been extended, and the effects of exchange and of spin—orbit coupling have been qualitatively considered. Three models are discussed with varying relative importance of spin—orbit and crystal energies. These models are correlated, and the strong crystal force model is used to interpret the experimental observations.