The systematic preparation of synthetic peptide combinatorial libraries (SPCLs), each composed of tens of millions of peptides that can be screened in existing diagnostically or pharmacologically relevant in vitro assay systems, is reviewed. The identification of optimal peptide sequences has been achieved through the screening in solution of SPCLs, each element of which is composed of more than 100,000 nonsupport-bound peptides in equimolar representation, along with an iterative synthesis and screening process. Examples are presented in which an SPCL, composed in total of 52,128,400 acetylated hexa-peptides, is used along with an iterative selection process to precisely identify the antigenic determinant of a peptide recognized by a monoclonal antibody using competitive enzyme-linked immunosorbent assay. This same library was also used to develop highly potent antimicrobial peptides in bacterial growth inhibition assays. A separate non-acetylated SPCL was used to screen and identify high affinity peptide ligands using an opiate radio-receptor binding assay.