The invention provides methods for screening multimeric antibodies produced by mammalian cells to find those that exhibit a biological function. The methods can be used to screen large numbers of antibodies, which may be cell surface, secreted, or intracellular antibodies. Antibodies can be screened to find those that bind antigen more avidly or those that compete with a ligand that binds to the antigen for binding. Any biological function that can be tested in vitro can be used to screen the antibodies. Nucleic acids encoding the antibodies that exhibit the biological function can be obtained in a number of ways.