Delta-aminolevulinic acid dehydratase (ALA-D) is a enzyme that participates in the pathway from 15-aminolevulinic acid (ALA) to porphobilinogen (PBG). This enzyme is relatively easy to analyze because erythrocytes can be utilized as the sample. However, it is not completely understood what kinds of abnormality in porphyrin metabolism influence ALA-D activity, and it has few clinical applications.To elucidate the regulation of 15-aminolevulinic acid dehydratase (ALA-D) activity, we compared the ALA-D activity and protoporphyrin levels in erythrocytes among three different species. The values for erythrocyte ALA-D activity were highest in the guinea pig, followed by man and the mouse. Erythrocyte protoporphyrin levels showed a similar tendency. A definite correlation exists between ALA-D activity and protoporphyrin levels in erythrocytes. From this data, it is speculated that ALA-D activity in normal metabolism is a marker of heme synthesis.