Using string as a lock to do thread synchronization

While i was looking at some legacy application code i noticed it is using a string object to do thread synchronization. I'm trying to resolve some thread contention issues in this program and was wondering if this could lead so some strange situations. Any thoughts ?

private static string mutex= "ABC";

internal static void Foo(Rpc rpc)
{
    lock (mutex)
    {
        //do something
    }
}