Immunotoxins, consisting of antibodies coupled to toxins, are extremely useful tools in the elimination of specific cell populations in vitro and in vivo for research and therapeutic applications. The antibody is used to target the toxin to a specific cell population, which is distinguished by its cell-surface antigen. Not all antibodies are suitable for creating an immunotoxin, and large numbers of antibodies may need to be screened. This is a time-consuming and expensive process if each potential candidate must be conjugated to the toxin and purified. A faster and more economical way to identify potential targeting antibodies is to use a second immunotoxin, an anti-IgG antibody that is coupled to the toxin. The second immunotoxin eliminates the need to couple every candidate antibody to the toxin because it can simply be added to cells in culture with the antibody of interest. Using this method, many antibodies can be screened quickly and efficiently for their ability to internalize.