The subjects of the invention are kits comprising free or support bound peptide mixtures (sub-libraries) as components, methods for their preparation and method for their use in discovery of bioactive peptides as potential drugs. Groups of free peptide mixtures (sub-libraries) forming components of the kit may carry different color or fluorescent labels facilitating their identification. The solid support in kits comprising support bound peptides may carry color or fluorescent labels as well, but the size or the specific gravity of the beads are also used as markers. The kits can be applied in all kinds of biological screening tests, including binding tests with macromolecules (like proteins), cells or tissues. The amino acid sequences of bioactive peptides can generally be determined in three stages: (i) determination of amino acid occurrence list by use of amino acid tester sub-kit, (ii) determination of occurrence library by use of the first sub-kit, and (iii) determination of sequences by use of second sub-kits.