The rapid quantitative microfluorometric assay for free erythrocyte porphyrins can be applied in the diagnosis of erythropoietic protoporphyria. The advantages of this assay are speed, accuracy, reproducibility, and lower cost, as compared with other biochemical methods now in use. When local facilities permit, this assay can be done on a wet blood specimen. Otherwise, drops of blood from the finger can be collected on filter paper that can be submitted by regular mail to a laboratory for quantitative analysis by this assay.