The present invention provides peptides libraries which are useful for rapid identification of biologically active compounds. The invention further provides peptides which include cell-growth affecting peptides and peptides which enhance or inhibit production of cellular proteins. Many of the peptides of the invention may be produced in large quantity by recombinant techniques and formulated in culture medium to produce the desired effect on cultured cells and tissues. Certain of the libraries of the invention and the peptides identified in them are particularly useful in concatemer-based recombinant expression methods.