Abstract 1. 1. Two new procedures are described for the quantitative assay of “free” and zinc-protoporphyrin in red blood cells. 2. 2. Major clinical findings have included the following: (i) An average of about 10% of the total erythrocyte protoporphyrin is “free” in normal humans, but about one-third or more is usually “free” in blood of normal cows, rats and rabbits, (ii) In addition to human and bovine protoporphyria, “free” protoporphyrin was increased in (a) genetic carriers (clinically normal) of bovine protoporphyria, (b) acute Pb poisoning, and (c) some patients undergoing dialysis, (iii) Significant amounts of zinc-coproporphyrin were found in the blood of several dialyzed patients and in some “normal” cows.