Random peptide libraries and antigen-fragment libraries (also known as gene-fragment libraries) have been used to identify epitopes on protein antigens. These technologies promise to make significant contributions to diagnostic and vaccine development. Researchers in a number of labs have shown that phage selected from libraries with protective antibodies, raised against whole antigen, can be used as immunogens to stimulate antibody responses that bind native antigen and provide protection in vivo. Others have used the sera of patients with idiopathic diseases to screen libraries, and by this approach have identified candidate antigens involved in immune disease. These may prove useful for diagnosis and, possibly, in determining disease etiology.