Using precipitating antibodies to ACI rat liver ferritin and to sodium-dodecyl-sulfate-dissociated protein subunits of ACI rat liver ferritin, we have demonstrated the presence of ferritin-positive sites and subunit-positive sites in situ in several rat hepatoma cell lines by immunofluorescence. Hepatoma cells from three transplantable rat hepatomas (Reuber H-139, Reuber H-35, and Morris 5123) were explanted and propagated. Rabbit antibodies specific for either protein subunits of ferritin or ferritin were prepared by affinity chromatography or by dissociation of antibody-antigen complexes with 0.1 M acetic acid followed by differential ultracentrifugation. Explants of Reuber H-139, Reuber H-35, and Morris 5123 hepatoma cells, grown either in ordinary McCoy's 5a medium or in such medium enriched with iron (0.002% Fe), gave positive immunofluorescence for subunits as well as ferritin. Exposure of a clonal strain of Morris 5123 hepatoma cells to iron-enriched culture medium for varying lengths of time of up to 24 hours resulted in progressive increase in the quantity of ferritin-specific immunofluorescent cytoplasmic material, which was at first present diffusely, and later in clumps. By contrast, during the initial 24-hour period, subunit-specific immunofluorescence remained at relatively low intensity, with diffuse distribution through the cytoplasma. Our findings indicate a) the presence, in the cytoplasm, of the three kinds of hepatoma cells, of unassembled or only partly assembled subunits of fragments of subunits as well as of ferritin, and b) rapid assembly of the protein subunits into apoferritin and ferritin after administration of iron, so that the concentration of subunits in the cytoplasm was not significantly increased.