β-Phenethylbiguanide (DBI) was found capable of forming chelates with copper and nickel ions, but no cobalt, ferric, ferrous, zinc, or calcium chelates could be isolated. A 1:1 copper-DBI chelate was formed in acid media, and a 1:2 chelate was formed in neutral or basic solution. At pH 4 where both copper complexes existed in equilibrium, DBI was found to have an affinity for copper ion of the same order as glycine. DBI is also capable of chelating with the 1:1 copper-glycine complex. Among a series of alkyl-substituted biguanides, some correlation between ability to chelate copper ion and hypoglycemic activity was observed.