Background: Accessories, watches, coins and other items containing metal sometimes cause contact dermatitis and metal allergy. Among metals, nickel in alloys is ionized by sweat on the surface of the skin and exhibits particularly marked irritancy and allergenicity. Although eosinophils play important roles in allergy, the effects of nickel on eosinophils have not been elucidated. Methods: Eosinophils were prepared from the peritoneal cavity in rats immunized with Ascaris suum extract. Purified rat eosinophils were incubated in the presence of various kinds of metals including nickel. The viability of eosinophils was analyzed using a flow cytometer. Results: When rat eosinophils were incubated for 3 days in the presence of nickel chloride at 30-1,000 μM, the viability of eosinophils was decreased in a concentration-dependent manner. Nickel chloride at 300 μM significantly increased the percentage of annexin V+ PI- eosinophils. The population of annexin V + PI- eosinophils was also increased by nickel sulfate, cobalt chloride and zinc sulfate. The binding of nickel ions to eosinophils was detected by flow cytometer. Conclusions: Nickel ions bind to eosinophils and decrease the viability of eosinophils through the induction of apoptosis. Nickel ions may exhibit activity which modifies the function of eosinophils in allergy.