The present invention relates to novel peptide ligands for prostate specific antigen (PSA) binding specifically with it and enhancing its enzyme activity, to a process for preparation of these peptides, to diagnostic and pharmaceutical compositions comprising these peptides, to the use of these peptides for pharmaceutical and research preparations, to methods using these peptides in diagnostic assays for determination of the concentrations of various molecular forms of PSA, to methods for modulating the PSA enzyme activity and PSA activity dependent conditions by using these peptides either in vivo or in vitro and to the use of these peptides in procedures for biochemical isolation and purification of PSA.