Available data from human subjects suggests that most of the pools of free protoporphyrin in circulating erythrocytes have T 1/2 values which range from less than 1 hour to approximately 2 weeks. An exception was observed in 2 cows with erythropoietic ("congenital") porphyria. During the several months which followed the simultaneous injection of 14C- and 3H-labelled glycine and ALA, changes in the specific activities of both free- and hemoglobin-protoporphyrin were similar. Since erythropoietic porphyria in the bovine differs from that in the human mainly in the elevated values of free erythrocyte protoporphyrin in the former, it is suggested that this prolonged T 1/2 (equal approximately to the red cell life span) may account for the elevated values seen in the bovine. Variable ratios of red cell/fecal protoporphyrin concentrations reported in other diseases are also believed to be due largely to T 1/2 differences in porphyrin pools. Recorded fluorescence and excitation spectra of blood showed the single fluorescence band of "free" porphyrin in bloods of normal and porphyric cows and of patients with erythropoietic porphyria, protoporphyria, and sideroblastic anemia. The 2 band zinc complex spectrum was found, as expected, in blood of subjects with Pb poisoning and Fe deficiency anemia. T 1/2 values of 1 to 2 weeks were calculated for red cell protoporphyrin from previously published data of patients with Fe deficiency anemia who were treated effectively with iron.