We describe a radiochemical method for measuring ferrochelatase activity in the in vitro incorporation of iron into mesoporphyrin-IX to form mesoheme. 59Fe is used to quantify ferrochelatase activity in rat liver. The uptake of iron is linearly related to the time allowed for it to occur, and the reaction proceeds optimally under reducing and anerobic conditions, maintained in sealed lyophilization vials under a positive pressure of nitrogen.