Erythrocyte protoporphyrin concentration was determined on a single drop of blood from normal and anemic mutant mice using a modified hematofluorometer. Mice showed age- and sex-dependent changes in protoporphyrin concentrations. Mice with hypoplastic anemias had a slight elevation and mice with iron deficiency anemia had a moderate increase in red cell protoporphyrin. Those with inherited hemolytic anemias had a marked elevation of erythrocyte protoporphyrin. Mice heterozygous for hemolytic anemias had no overt hemolysis and reticulocytosis, yet their erythrocyte protoporphyrin concentrations were distinctly higher than their littermate controls. Previously these heterozygotes were differentiated only by laborious and expensive progeny testing. This paper is the first demonstration of a quick and practical way of detecting mice heterozygous for hemolytic anemias by erythrocyte protoporphyrin determination. The data of this study demonstrate not only the usefulness of the hematofluorometer for protoporphyrin determination in the diagnosis of mouse anemias, but also the significance of elevated erythrocyte protoporphyrin as a very sensitive indicator of increased erythropoiesis, or of iron deficiency.