Iron metabolism during inflammation was studied in normal and stimulated peritoneal mouse macrophages. A model was developed for detection of ferritin synthesis in these cells, and inflammatory stimulation was produced by i.p. injection of thioglycollate broth or i.m. injection of turpentine. The cells were adhered to culture discs and incubated at 37 degrees C with 59Fe-transferrin and 3H-leucine, and after washing the cell lysate was chromatographed. In the lysate from stimulated macrophages iron and tritium activity was found in a joint peak, and both were precipitated by a specific anti-mouse ferritin antibody. No significant peaks of radioactivity were found in lysates from normal cells. This showed that inflammatory RES cells have an increased ferritin synthesis. The uptake of 59Fe was investigated in a separate study and the stimulated macrophages were found to have a much higher iron uptake than normal macrophages.