The presence of an iron-containing complex (FSC, ferric sorbitol citrate) in medium inhibited proliferation of malignant (KB. GHC) cells: but did not appreciably alter the proliferation of normal (HBS, Vero) cells. Flow cytometric analysis showed considerable differences of malignant and normal cell growth kinetics. With addition of 200 microM Fe in FSC, malignant cell proliferation was suppressed. An increased number of cells in G1 and early S phase suggested that iron excess blocked the cell cycle before beginning of DNA synthesis.