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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.