SummaryAdding ammonium ferric hexacyanoferrate (AFCF) to cows' fodder produced after the Chernobyl nuclear accident prevented milk contamination by increasing the faecal elimination of 137Cs. Synthesis of ammonium ferric hexa[14C]-cyanoferrate (AF14CF) and its purification were performed for the study of the metabolic fate of this complex, and the evaluation of the possible release of cyanide. The stability of this colloidal product, tested by anaerobic incubation in rumen juice in vitro, showed no release of free cyanide from AF14CF, but hexacyanoferrate was identified in the rumen juice and 0·13% of the added radioactivity was converted to labelled CO2. AF14CF administered per os to two cows showed a nearly quantitative excretion of radioactivity in faeces during the first 3 d (91–95%). A very low but significant level of radioactivity appeared in plasma, blood cells, expired CO2 and was detected in organs taken 9 d after administration. Total cumulative radioactivity in urine and milk amounted to 0·19–0·47% and 0·068–0·071% respectively for the two cows. Labelled hexacyanoferrate and thiocyanate were identified in the urine and also in faeces. In spite of this relative instability of AFCF in the rumen of cows, the poor absorption of AF14CF degradation products showed that AFCF constitutes an efficient and safe food additive to prevent the absorption of radioactive caesium from ruminant feed and its secretion in milk.