System.Collections.ArrayList arr1 = new System.Collections.ArrayList();
System.Collections.ArrayList arr2 = new System.Collections.ArrayList();
arr1.Add("1");
arr1.Add("2");
arr1.Add("3");
arr2.Add("1");
arr2.Add("4");
arr2.Add("5");

for (int i = 0; i <= arr2.Count - 1; i++)
{
if (arr1.IndexOf(arr2[i]) == -1)
{
listBox1.Items.Add(arr2[i]);
}
}