The soils of Bikini Atoll (11 degrees 35'N, 165 degrees 25'E) were contaminated by fallout from a thermonuclear explosion in 1954. Today, in the absence of any treatment, intake of 137Cs via the terrestrial food chain could account for 70% of the radiological dose received by a returning population. Therefore, we examined the effectiveness of potassium applications, alone and combined with nitrogen and phosphorus, in reducing 137Cs uptake by coconut (Cocos nucifera L.), a major food crop. Mean pretreatment concentrations of 137Cs in "drinking-nut" meat of approximately 17-y palms ranged from approximately 0.5 to 7.0 kBq kg-1 (approximately 14 to 190 pCi g-1) (wet mass) in the seven primary experiments reported. These values were reduced to approximately 0.06 to approximately 1.0 kBq kg-1 (approximately 1.6 to approximately 27 pCi g-1) following soil additions of potassium-chloride at rates from 670 to 6270 kg potassium ha-1. Major reductions were complete within 9-12 mo after single large applications and persisted for at least 3 y. Proportional reductions in associated drinking-nut fluid and in mature "copra nut" meat from the same palm also occurred. Comparable but shorter-lived reductions occurred in grass and herbaceous species beneath the palms. A combined nitrogen-phosphorus treatment had no additive effect in the presence of potassium but, by itself, reduced plant uptake by about 50%. The persistence of this effect suggested that phosphorus alone was the major factor. Periodic addictions of potassium at rates of approximately 1000 kg ha-1 would provide a feasible and highly effective means of reducing 137Cs in coconut food products.